Disc wear depends on how and where you drive. From the stuff on this site, US drivers get much higher mileage from their brakes that we do in the UK. I guess it's down to longer, straighter roads!
A rule of thumb is new discs every three sets of pads.
Warping is due to heat. Main factors are
1) Heavy braking generates lots of heat
2) Worn discs can't dissipate the heat and warp easier than new discs.
3) Poor quality discs will warp before good quality ones. This is largely down to quality of materials and heat treatment of the disc during manufacture to relieve stress.