How much can I safely prune off a grapefruit tree?

All Citrus have the unfortunate in built problem in that they store very little food reserves in their stems and roots so that if you prune them too hard back, they don't have enough energy to re-shoot. My only advice is to either prune out the odd stem or take back shoots no further than where there are still leaves attached to a stem.

Thanks Fractal, your advice was very helpful