depends on the seats... but for some regular bucket seats, it shouldn't take more then three and a half yards of fabric for each of the driver and passenger seats. You'll probably want to pick up...
i work at an upholstry fabric store and we send out probably 5 items a day to be reupholstered, our main sources say around $250 for labor on a wing back chair, and around 6 yrs of your own fabric....
My quasi-educated guess would be around 7 yards. Depending on the size and style of the chair, it could be as low as 6 yards or as high as 8 yards.
If the fabric has a repeating pattern or the chair...