Not a vet but have seen 7-10 day trials at farms. I've seen longer (2 weeks to a month) at other farms that are used to leasing horses out. Don't know how long meds can stay in the system, but I would have written into the sales contract that the horse can be returned within two weeks for full refund if temperment doesn't prove as advertised (along with soundness). It goes without saying that a complete vet check prior to purchase is requisite. You can also check with your vet about what kind of bloodwork you can have done to check for tranqs and pain killers. I would be concerned that joint injections may mask arthritic conditions and would ensure that joints and hooves are xrayed.