In the UK, pubs open at 11 (or used to) so that's a reasonable hour
. I tend not to want to stop drinking beer once I've started (I don't like the taste of anything else afterwards, so when I want another drink, it's beer again (and it has to be the same brand too -shit, I AM weird, it becomes so much clearer as i write it down)), so I generally tend not to start until the evening, but have never made a 'rule" because I suspect the things about rules are (a) they're made to be broken and (b) they tend to start to work both ways -if you have, say, an 8pm rule, you might start to feel you should be drinking if it's after 8, even if you don't actually fancy one? or feeling that once it's past they hour you can cram in as much as you want? maybe I protest too much.
My mom (who drank way way too much) had a "sun's over the yardarm" riule, which apparently meant about 5pm. Although I later learned that was just for driinks that actually looked like drinks. if it could fit in a glass of tomato juice or a cup of coffee, then any time was fair game
