depends on the car, its just taken me the best part of 6 hours to replace the front left bearings on the midget, and thats with all the correct tools! Some cars just dont make the job easy.
Remove wheel
Remove brake calliper (PITA as the safety retaining tabs seem to be the most solid part fo the car!)
Remove hub-cap and nut
Bearing puller to remove hub from axel
Locate suitable drifts for removing bearings.
Strip all the bearings out (one cartridge and a seal on each side).
Degrease everything (this can take a while)
Locate suitable drifts and spacers for pressing the brarings in, this takes a while.
Press bearings in (this takes a while, the tolerances are much much tighter than your average MTB hub, even with a press this takes some serious brute force)
remove disk from hub
wipe off any remaining grease, remove gloves (its easier to know if your getting grease everywhere without them)
Install new disk,
Press hub back onto axel and repeat the removal process.
On the other hand it could just be a needle taper roller bearing and will take you next to no time.