In my experience, I ended up waiting a whole morning or a whole afternoon for a 30-min or 1-hour gliding flight. Not really the best use of my time. If I could do it again, I'd rather put the money on powered aircraft.... with most schools, you turn up, have a short wait, brief, fly, land, debrief, and you go home. No time wasted sitting around or helping out. Not that there's anything wrong with that, if you have the time to spare.