Considering the 65.5k(ish) character limit, 15k word posts are pretty rare. But posts approaching that area is entirely possible. Especially for the higher advanced collabs. When we transferred our advanced star wars RP from backups to the new guild there was at least one post that exceeded that character limit. 

RPs based around longer collabs do tend to be slower, yes. But there's still stuff happening in there. The worst thing for an RP is if two people base around long collabs, while the rest base around quicker posts and neither party is willing to adapt. Thankfully, that is rare.

So far as long posts go, you can't have them be too long if only one user is involved, as that will in 90% or so of cases leave little room for interaction. It takes a truly well-written post to allow for good interaction if it is in the several pages area of length.

Some Advanced RPs can seem pretty dead, then suddenly have several posts in a couple of days. Ours had about three weeks with no inactivity, then suddenly four massive posts in 24 hours. So yes, it can happen.

One thing we've heard commented about ours is that unlike many it actually welcomes people that aren't used to writing long collabs. We don't really know why others don't allow such too. It never hurts to be helpful or open...