Thousands!
My son is at Uni and rang me last night to announce he's packing up drinking completely. He's not much of a drinker anyway (neither of my kids are, unlike me at that age), but went out on a social night with the footie team and because he was 20 minutes' late getting there he had to do a forfeit which involved drinking vodka. He said he felt ok going back to halls, but felt rough the next day (and as he is a bit of a fitness freak - not muscle man gym freak, but cycles, runs, plays football etc) and said he absolutely hated it and he just couldn't understand how some of his friends could do it night after night.
We had this great long chat and he told me how he doesn't understand why people want to go out and get hammered all the time, and how he just prefers to go for a meal with friends or watch a film or go to a gig. He also doesn't get the whole 'clubbing' thing - loads of pissed up and drugged up people crammed into a dark room dancing to shite music.
It just made me think of the decades I wasted getting pissed up, the money spent, the irresponsible things I've done (one night stands) the booze calories I've packed in (and had a lifelong battle with my weight as a result) the zillions of fags I smoked, the time I've wasted (out on the razz and days lost to hangovers) the idiots I've got myself hooked up with (some of which turned into longer term relationships - arseholes all of them) the toll on my health, wealth and reputation. I'm never going to get those years back that's for sure. I only managed to pack up smoking about four years ago, and gave up the booze this year (although have slipped up a bit recently), and started going to yoga etc and trying to improve my health and fitness.
I remember going out clubbing one weekday evening, meeting some bloke, staying out all night then going into work next morning still half pissed (and it was a well paid responsible job) wearing the bloody black velvet knickerbocker suit and a frilly shirt I'd had on the night before (well it was the 80s). My toes curl just thinking about it! 
So I wish I'd become disenchanted with the party lifestyle much sooner and 'grown up' while I still had options to do something with my life. Instead I'm looking at a skint and lonely old age unless an elderly millionaire chances by or I win the lottery. No pension, rented house, barely scraping by most months. Although I am trying to do something about it now ... in reality I've left it too late. I look at my kids starting out as adults and there's me looking at getting my bus pass) and I just wish I'd made more of the opportunities I've had - and yes I've had them and wasted most of them. I had looks, brains, and plenty going for me .. but I squandered it on having 'a good time' - which wasn't really a good time at all!
I also wish I'd had more respect for money and not allowed my ex (we weren't married) to do one over on me financially so I walked away with literally nothing, even though I'd had my own house when we met.
So yeah, lots to regret, but nothing I can do about it now other to accept my lot and get on with it, and hope my kids don't make the same mistakes I did. They seem a bloody sight more mature and responsible than I ever was, thank goodness!