I tried this back in 2016 and wrote several posts about my experience. I think I was fairly scientific about it, and tried it for long enough to get a good idea of the actual results and why they were occurring that way.
In May I tried this again, but using higher quality, fresher, whole seeds. I ground them myself, as fine as possible using a granite mortar and pestle, right before use and I kept the seeds stored in a cool (~70° F), dark, and dry place, which is important to maintain the quality of most spices.
During the three weeks in which I tested I used two different methods to steep the seeds. First by just adding 1.5-2 tablespoons of the crushed seed to cold water. I did this for varied lengths of time but found that 10-15 minutes seemed best, I can't really explain why. I also used the same process but used boiling water poured over the crushed seeds and then steeped, using a sealed mason jar for both temperatures. I did some experimentation with longer time periods, even several hours, and I wouldn't recommend doing that.
The first problem is that even with a granite mortar and pestle that weighs 12 lbs and that I can put my full body weight into, these seeds can't be crushed into a water soluble powder, and the shells or husks will flatten but not break apart. This means the drink is very gritty. Gritty probably isn't the right word since the last mouthful is just wet seed husks and it is difficult to get down. I considered sifting out the husks with a sieve but I could not find any indication that they do or do not contain anything that might be related to the effect.
The taste of the drink is pretty bad. Kind of like old grass clippings, but it also has a bitterness that reminds me of some types of cactus. It's tolerable, but barely when you consider the seed husks too. Both of these would be annoying but would be a daily chore I wouldn't really complain about if it had the effects that people claim.
In 2016 I noticed an effect almost immediately. If you're interested you can read about that experience earlier in the thread but to summarize I saw a rapid increase in libido, stamina, the volume of precum and cum I produced and the intensity of orgasm. That continued for awhile then went away. My conclusion then was that it was a placebo effect.
This time I took the first dose at about 10 PM at night and had no noticeable effect that night. The next day I took the dose once in the AM and again in the PM. On the first day of the full dose I felt like my libido was increased, however, I feel like some other behavior I was already engaging in had begun that trend. I did edge a lot that day and the following two days. I shot huge loads (for me) each day and had more precum than usual. At one point during those first couple of days I considered stopping because the amount of precum was actually very inconvenient and I found myself having to wipe off my thigh even when I wasn't really doing anything sexual. It even got annoying while I was masturbating because of how messy it was getting, but if I had been in a different place then maybe that could have been a lot of fun.
Over the next couple of days the effects diminished and by day 6 or 7 there was no real effect from the supplement, aside from the "side" effect that celery seed has actually been used for thousands of years for which is increased urination.
While I was researching this I found that celery seed has been used medicinally as a diuretic in primitive medicine for probably thousands of years. A diuretic basically makes you urinate a lot, but it also makes all the parts of your body that leak, sweat glands, tear ducts, etc., more leaky. So when you notice an increase in any fluids being produced its really just that your body is jettisoning fluid at an accelerated rate. If you produce a larger volume of semen or precum, its really just that your body has supplemented the normal supply with the hydration that your body normally wants to retain. I guess who cares, but you should probably notice thinner loads if anything, not thicker. If your loads get thicker then you should keep an eye on your prostate.
I think there are a few things that might make this work, sort of, but I don't believe any of them would typically have a long term impact.
-Celery seed is a diuretic and causes the body to produce retain less fluid generally. This should only have an impact until your body is dehydrated and begins retaining fluid again, or some degree of homeostasis is reached which should normalize fluid retention.
-Celery seed contains zinc and magnesium. Both of these supplements are involved in the production of seminal fluids and there are other threads on this board in which some claim similar effects when using these as supplements. The side effects of these supplements make their use infeasible for anyone who isn't deficient in them, terrible headaches, vision problems, etc. But if you are deficient in zinc and or magnesium then supplementing with them, or with something that contains them like celery seed, can lead to an increase in production. This probably shouldn't be considered an increase in functionality, but rather the removal of a roadblock to that functionality.
-The placebo effect is real but is not tied to any physical difference in performance. Instead, you are more stimulated because you are exploring a new sexual thing and our brains reward us when we do that, more than almost anything else. This is how taking a supplement, or trying some weird device that claims to do something, can seem to have an immediate effect but dissipate a while later.
I think this can be a fun thing to play with from time to time, but it doesn't work long term and probably isn't great to consume long term either. Though, there are benefits to taking diuretics and I did feel kind of flushed after using it for a short time, whatever that means.
Supposedly cultures that have diets that include diuretics tend to be healthier, thinner, and live longer. I don't know if there's any real correlation, but they also just consume it at a normal rate and don't try to overdose on it like this method calls for.
I'm done taking it for this purpose. The best use I've found for it is to put about a 1/4 teaspoon in with some broccoli while its boiling.
TL,DR
It might work for a few days and then will stop. The exception is, I think, if you have a zinc or magnesium deficiency.