How long does it take for the giant camps to respawn with new enemies?

10 in-game days (or 30 in-game days if you have 'cleared' it) of not entering the cell.

There is a serious game breaking bug involving respawns which bethesda have yet to acknowledge. One of the previous patches introduced this bug.

If you kill an enemy like a giant then revisit the area between 2 and 9 days later you will find the bodies have disappeared but the side effect of this is the respawn timer totally breaks.. After this no amount of waiting will allow the enemy to respawn.

This is why if you travel the same areas a lot you may find giants never respawn in certain areas due the the respawn timers breaking when there corpses disappeared.

This bug effects the 360 version. Not sure if it effects the ps3 or pc version.