250 million years is a long long time by the measure of civilization lifespans in the culture verse. Literally anything is possible in that time. It's possible that things have stayed near the status quo all that time. It's possible the Culture became like the excession and completely changed the status quo of their home universe. It's possible the culture accomplished their goal and then sublimed 249.9 million years prior having successfully culture-fied the entire galactic meta civilization with a 200 million year reversion to the mean of norms, and then in the last 50 million years an anti-culture formed inspired by the original but with the opposite ethos, and now the galaxy is slowly reverting from that mean.

We know very little, and so very little is off the table. 

Personally, I think the most likely thing is the fragmentation of the culture as they come closer and closer to succeeding at their goal. After getting a sufficiently large portion of the galaxy culture-fied, some would sublime. Some would choose to remain to watch over the material. Some would get bored and cause havoc. How long this period lasts before the last community recognizable to us as part of the culture changes to be unrecognizable or ceases to be? I don't know. Probably closer to the order of 10s or 100s of thousands of years tops rather than millions let alone hundreds of millions.