Looking at the current poolsize is only 33,506, and the target is 40,960 so we are still under size. The stake difficulty won't be increasing any time soon! (482 more blocks at least, as you only go up 15 tickets / block.)
It seems PoS diff adjusts every 800 blocks, it just adjusted to 2.57 on block 4896 which is 800 blocks after 4096.
Hmm I can't find any reference to 800 blocks in the code or params, I think that's probably just a co-incidence given the window size and number of windows, combined with the history of the ticketpool?
It adjusts every 144 blocks based on the exponentially weighted average of the adjustments scheduled around the number of new tickets purchased per block and the size of the ticket pool.
I think we could see PoS difficulty in the high 20s to maybe even 30 something. Why you say? Simple. 1813312 (current mined amount total) / 45 = 40295.82222222222 (41k being the approximate pool size.) That would only be if everyone was staking DCR though and not everyone is. So I think 20-30 is more probable right now before it stabilizes. Then it will slow down once more people need free DCR to do transactions rather than stake their wad.