GM Advice: How do you translate 'sessions' to PBP-length?
I don't bother with it, and here's why--
In any given group I played with, how much we accomplished in a session depended largely on who was there and how much we felt like socializing and editorializing on each others' actions. I got into role-playing for the same reasons a lot of people used to play Bridge...it was a good excuse to sit down with friends and enjoy each others' company for a few hours.
So, on some nights, we might get really swept up into the game, make some lucky choices that short-circuited some of the adventure-as-written (our GM was a firm believer that if you were smart enough to find gaps in the writers' plot, you should see the benefits of that...but he also found ways to attach downsides to it, later). In other gaming sessions, we might get maybe an hour of actual gaming fit in amongst two hours of socializing and storytelling, making bad jokes, etc.
It is, perhaps, easier to consider PBP adventures in 'chapters', similar to what BlessedOracle mentioned...pick out milestones, the kinds of places you'd regard as natural breaking points in the story (either for the cliffhanger effect, or because it was just a solid point of transition where the party had done what they needed to do and it was time to move on to somewhere else...something like that). Use those as your 'session' markers...