There are no set rules; its whatever both parties decide on before entering the loan arrangements. Whatever both parties feel is fair for the given arrangement should be drafted up as a contract before any animals are exchanged.
I just finished a breeding loan with a member on this site and I had a great experience. We agreed in writing to split the clutch 50/50 with him getting first pick.
I think this may not be the way we end up splitting things up but its always good having a baseline in writing to revert to if any disagreements arise to what is fair.