I have quite a few ball pythons now and I usually only give them about 30 seconds to a minute max. Then I'll note it down in my log book and move to the next snake. Sometimes I'll go back with frozen thawed one or two more times for a quick try and sometimes they will take it, usually not. Then I'll follow up with a live rodent and they usually always take that one. You can sometimes try touching the rodent to their side for a second or two. Sometimes I've had luck actually dropping the rodent accidently and that's when they grab it LOL.