The last time I bottle conditioned my Imperial Stout, it was flat as hell at 4 weeks. Leave it at room temp, and just forget about it. 3 months after bottling, it was the best stout I'd ever brewed. If you're priming a high abv beer, with flocculent yeast such as a dry english or california strain (as I did), and holding temp at 65, it will take a little time. Be patient. RDWHAHB.