there are no cut offs, it's a linear increase in %cp lost per hour till 1300ish, and exponential curve till 2400ish then linear again after that. motivation or evolution level have no bearing. higher the cp, faster it gets to critical levels.
so basically you have to balance cp vs. how long you expect it to be in gym. i do gyms very late at night so i've switched to chanseys, miltanks, wobuffets and azumarills since i know it won't be attacked for at least 6 hours - my 3k+ blisseys will be sub-1k and probably only need 1 fight to knock out but things i mentioned will have at least 2, possibly 3 fights in them. alternately if i expect to be attacked very soon (e.g. holding down a gym before a boss hatches) i roll out the big guns.
assuming ofc you aim to hold gym as long as possible, which is pointless currently as higher the cp more chance a 'mon will last enough time to gather coins but not long enough to keep you from collecting them. depends on your local scene.
