To answer your question "how long", it depends on your site size, quality, consistency, and of course, popularity. Of course Google does what it wants too, so it really just depends on how it may feel that day.
Case 1) Popular forum with 1000+ avg 30 min sessions, above PR 5 --> Sitemaps are crawled and indexed multiple times a day, sometimes multiple times an hour. Its not uncommon for your posts/users on these forums to show in Google results within 10 minutes.
Case 2) Content rich forum with relevant targeted data, spam free, above PR 2 --> Sitemaps are crawled multiple times a day, up to once an hour, however its up to Google whether it indexes them. On a certain small fry forum we work with, ive seen posts appear in Google results in under 4 hours.
Case 3) Other small, off the radar forum, with diluted content, spam target, or too much competition --> Sitemaps are crawled a couple times a day and if it doesnt see changes happening, or no users are querying Google for these SERPS, it may take up to a week or more to index.
Note) Even if your sitemap errors are repaired, RDF is a different animal - unless you are a super popular site, it can take up to 2 months to fully index the widgets. Its even longer to forget if there is error - errors from RDF may stick for 8+ months (although often disabled in index immediately). If you fix your sitemap errors, and things still aren't being crawled right, look for any RDF error in webmaster tools > search appearance > structured data
Hope that helps!