First off if this is for writing and this is just my opinion. Try not to get lost in how long two women fight for, reason being if you limit your “time” to get to an ending you might miss the chance to add some move or counter move or other avenue the characters could have happen. Also take into account if you are talking a real spontaneous catfight, you really can not get an accurate time, mostly because i would say 97% of the time if not more, when two women are going at it, there are two men at the least there to rush in and break it up before one woman screams her surrender, mostly cause of the amount damage two women are doing to each other. If you are thinking a rules arranged fight /match it comes to two ways the women will agree to, 1) a time limit either per round, or match time 2) till one runs out of gas and or is put into a move that she can’t free of and quits. These can range from 5-10 minute rounds or as i said till. One gives. As far as things like breastfights if it is a no hands breast to breast it can well into an hour, with the first 15 minutes having little to no effect on either woman other than the arousal of the act. Factors include size, weight, firmness and a few moves that may or may not speed things up a bit. If the two women do a hands on style it will be a much faster match for the obvious reason of pain . A huge thing to remember if you never wrote before, what you think is a silly idea, or lengthy topic, others may fall in love with the idea, the character or the quality over quantity .