I'm no expert, having only done them twice now, but I reckon you shouldn't have to reduce the weight you'd normally load up - at least after a warmup set or two - and might even be able to squat a little bit more in your first session.
The time you spend seated is dependent on how long you want to hold that tension in your core before standing up again; the longer you leave it, the weaker you'll feel when you then try and stand, and there will be even more propensity to lean forward as you rise.
W.A. AMATEUR STRONGMAN
Axle clean-press: 100 kgs (220 lbs)
Bench press: 135 kgs (298 lbs) - 1st PL meet 16th October 2011
Deadlift w/Barbell: 180 kgs (397 lbs)
Deadlift w/Hexbar: 225 kgs (496 lbs)
Farmers walk: 240 kgs (530 lbs), 50 feet
Front squat: 100 kgs (220 lbs)
Log clean-press: 100 kgs (220 lbs)
Strict OHP: 85 kgs (187 lbs) 3 reps
Tyre flip: 260 kgs (573 lbs), 100 feet