1.You can expect to need about 15 flights w/ a CFIG, plus about 10+ by yourself to be ready for the checkride. There is no written for PPL transitional pilots.
If you already have time in old taildraggers and especially formation flight, the time could be less.
4. On a good day, it depends on the pilot, now doesn't it?
You might stay up for an hour and a half. You might stay up for 5. You might stay up for 25 minutes on that sleigh ride back down to the runway. Depends on you, on lift, on luck, on lots of stuff.
I never had a problem staying up as long as I wanted on a
good soaring day. It's the marginal ones that require some skill for that.
But the club I was in limited flights to one hour on busy weekends, to allow fair access to the gliders.
IIRC what you need to learn is:
1> Ground ops (rigging, launching, and recovering gliders)
2> Flying on tow. This is a lot like learning to land the first time. It starts out feeling near impossible and then gets so easy you wonder why it seemed so tough. Basically you have to make the sailplane do whatever the tow pilot is doing.
3> Handling a rope break at 200 AGL. This is so easy I wonder why they waste time teaching it. (j/k about the wondering part)
4> Landing in (at the airport). There are pattern issues and a towplane to watch out for. You have to plan the approach well cause you only get one shot, but spoilers make landing so easy you'll wonder why they don't put them on most airplanes.
5> Flying in a gaggle (group of sailplanes under the same cloud) without scaring the other pilots.
6> Landing out (away from the airport when you run out of lift) and retrieval/extraction (getting the sailplane out of the farmer's field without getting shot or damaging the aircraft and loading it into it's trailer).
Notice that I didn't mention finding and maximizing lift, flying cross country, or general soaring techniques. That's because you won't necessarily learn this while getting a glider rating on your certificate. You can learn that stuff, but AFaIK it's not required.
My experience in a soaring club is that you will have to spend the better part of any day you choose to participate. Going with a commercial operation costs more but would likely eat up less time.