I think it depends on overall frequency of crawling rather than calendar time. I have one directory that I created in January, with 301s in place at the outset. Google still crawls the old URLs about twice a month. I know because I've been waiting for them to stop so I can clean out my htaccess. The last time I did this, msnbot or someone like them came knocking at the door within days-- literally-- after I'd decided everyone had accepted the changed URL so it was safe to drop the redirect. Grrr. But now they can bloody well take their 404 and lump it.