Personally if you are using a tool like Open Site Explorer and are already undertaking a link cleaning exercise. If you identify no follow links on spammy or dodgy sites then I would add to the disavow as you are doing this anyway.

I would rather clean all links as who know that Google will change next so I prefer to ensure everything is legitimate and as it should be even if our clients currently aren't being penalised.

I wouldn't go out of your way though to disavow these no follows if you are not cleaning your links anyway.

LIke others have said. It can vary from site to site. I have seen examples where it has been the next day and others 2-3 weeks later. Google typically deep crawls a site a minimum of once a week but it does not guarantee that it will crawl all pages when it does so it may have been missed in the latest crawls. I have found also, sites with sound internal link structures get more pages crawled at a time. I guess the google crawler likes a easy read of a site.

regards

Dave