It depends on the year, but it is generally a slow decline. I imagine that the river will start to drop off next week. When the river drops it will slowly drop out - I would bet on 3'+ through June and 2'+ through July. Also, it generally hangs out at certain levels when dropping as well. (There is generally a week or so when it hangs around 2.75' before continuing down.) It should (hopefully) stay in the 1.5'-2' range until September.
Also don't despair, while it is true Steven's Down, Rustic, etc are awesome at 4' and above and are less fun as the water drops. The narrows offers legitimately fun boating at very low levels. The whole stretch goes and is fun down to 1.3' in my book. Upper and lower narrows are good below that, but middles gets pretty thin.
The above is the long answer - the short answer is that Winter is long, and ideal flows are short - go boating. Any other activity you might want to do, you can do in the fall.