No one needs to have it in for the whole day, 'cause regular use in a couple or more "sessions" a week will do the stretching you need. The only reason you'd want to "wear" one all day is if you thought it would give you a sexual thrill to secretly squirm on it while you were fully dressed in public or at work.
I'm pretty sure that you'd get tired of this before the day is out unless it was quite small. Also, with this continuous stimulation and pressure the rectum creates it's own natural lubrication. Your sphincter might get sore or tired trying to hold this in all day. However, for sexual play sessions this can be part of the fun, so you're very ready for whatever or whoever is going to penetrate you after the butt plug has done it's job.
Finally, if you are a virgin with respect to bottoming, but you'd like to try it, using a butt plug or a dildo for a few weeks or months will train your muscles to relax or grip under your own control, rather than as reflex responses. This can make the eventual experience of anal sex much more pleasurable for first timers.
BTW, not only gay men enjoy anal stimulation. You may not be thinking of bottoming for a man, but you may want to incorporate this toy into your own sexual play with or without your girlfriend. Letting her use it on you might be exciting for both of you.
(always be clean and wash toys with antibacterial wash or bleach after use - and do not share them)