I'm Creating a home-brew system for PBP

Hi Guys

I'm currently working on a system for PBP, not ready to release the details now, but I wanted to open up a thread so that I can pose questions to the community that it's aimed at.

Now my first question is: How long should a Boss Fight last in PBP?

To be specific, how many posts should it take to finish, before the encounter feels grindy and boring for all involved?