I did just that two years ago and last year. Each time a midweek course (5 days) at a gliding club.
First year, 15 winch launches, no solo.
Second year, solo after +/- 15 winch launces, 7 solo flights.
I already enrolled in this years course again.
What to expect flying-wise:
- Rudder, rudder, rudder! And since you're constantly turning to catch/maintain thermals, more rudder, rudder, rudder!
- If you normally fly LHS, with left hand on the yoke and right hand on the throttle, you've got to switch hands. Gliders are steered with the right hand, while the left hand applies air brakes, trim and such.
- Completely new picture in the circuit. On downwind you think you're awfully low for a glide-in, and then find you need serious application of the air brakes on final anyway.
- No touch-and-goes. Every landing is a forced landing and you simply put down the aircraft where there's space. Never mind the five others that landed before you and are still sitting in the landing field.
- Winch launches are a real thrill but it takes a while before you can execute them properly and safely.
- Most (all?) gliders are taildraggers with a center mainwheel, requiring full stick back on/after landing, and require that you "fly" the aircraft in both roll and yaw until it comes to a complete stop. Non-steerable tailwheel, so aerodynamic steering only.
- Air brakes are fantastic. Much more effective than flaps.
On the ground:
Gliding is a team effort and a full-day affair. Whereas in powered flying you show up a few minutes before your lesson/rental block starts, in gliders you've got to help out the full day. So you've got to be there for the morning briefing (no briefing, no flying is the general rule), then help get the aircraft out of the hangar and towed to the launch point. You've got to help prepare the gliders, pull them into their launch position, hook up the cable, signal to the winchman, and keep the wings level until aileron control has been achieved.
After an aircraft has landed, you are supposed to drive the tractor/golf cart to the glider that just landed, and tow it back to the launch point.
And at the end of the day you are expected to help getting the aircraft washed and in the hangar again.
All these tasks are typically taught to you on your first day by the other glider pilots hanging around. So don't expect an easy day the first day.
Slightly more specialist tasks, such as manning the "control tower", reeling out the launch cables, and operating the winch are typically for experienced glider pilots, or people who have had some specialist training. But trust me, even without these jobs there's plenty to be done.
The difference between gliding and powered flying can best be compared to sailing and motor boating. With an engine you're going from A to B and other than departing A and arriving at B, there's not a lot of excitement in between. Without an engine you're going to be busy and excited all day, but at the end of the day you are still at the place where you started. Kinda pointless, but great fun.