I was never a premature ejaculator, usually lasting 5-15 minutes of actual penis-in-vagina action, 5 if in one position, 15 if we switched positions. Yes, occasionally I would only last a minute or two, but that was very rare.
I asked for and tried Cialis for erectile dysfunction, which is what all the ED drugs are designed for. Even then, it didn't happen all the time, but any 'sense' that it might happen, or any loss of sensation, was almost assuredly going to make it happen.
The 20 mg does was from the sample pack from my doctor and while it was fun to have a super hard, super stiff, hang-my-towel on it erection, it was really way too much. The flushing, stuffy nose and backache side effects weren't worth it. I then tried 10 m, then went down to 5 mg, as it really is only an occasional issue with me and 5 mg gets me good and hard enough that my wife notices and gets the benefit from!