I'm not multi-orgasmic, so my ideal length for actual intercourse is usually 10-20 minutes. It depends on the guys size though. With a really big one, marathon 30 minute sessions are just way to much. With a more modest sized penis, I like to go a little longer, but even then if I haven't gotten off in 20 minutes I'm probably not going to without some oral action. Including foreplay I would say 30-40 minutes would be ideal.