There really isn't a stated time limit per say just that by the time Natsu uses his more powerful stuff, he typically wins the fight and low on magic because he's been fighting for the whole arc up until that point.
He gained the ability to absorb dark regulus aka darkness manip in his fight with eclipse Leo, he initially had an issue with eating it as it made him temporarily sick, but he quickly overcame this issue and could use the darkness at will without issue now. Even if Natsu did have some issue with absorbing Meli's darkness or fire he could just pull what he did against Zancrow and empty his magic to eat inedible elements.
Revenge counter is a last resort for Meli, and by the time that happens Natsu would have enough left in the tank to counter it with his own big moves or use his powernull, i will say RC is a possible win con for meli as it's a devasting attack, i just think Natsu has better chances to win here.
As for stats, i've already mentioned why stats won't play much of a factor due to natsu's absorption, and Meli's mid regen can be worn down, if need be Natsu could go for full body vaporization like he did on Zeref
when Natsu absorbs an opponent's power for the first time, he typically adds their stats to his own if they possess a noticeable stats advantage. This happened with Laxus, Atlas Flame, Ignia, ect.