So if I understand correctly you are not asking for a charge on items but more of an enhancer type item that once used will last for 5 battles whether you continue to use the item or not, so the timer is by battle and not by times used?
If so that isn't that hard, just create a troop event(in every troop) that activates by a switch being on and has a span of battle. Underneath it use a conditional branch that variable ==5 and if true add your state and remove the item or anything else you want to happen when 5 battles have been fought.(be sure to rest variable to 0) On the else branch just make your variable += 1 and if it isn't valid it will just add one battle to your counter.
Then of course it's just a matter of hooking up your item to a common event that turns on said switch, not sure about checking for escape though, if your battles are touch encounter you could cover it on there event pages, random not so much control though.