Are you hoping to gain PR juice or traffic or both from this sort of link building?

Making good posts in a good spread of other forums in your niche that permit the use of a signature.  I'd say you could knock out about 25 solid threads in an hour.  If you did this across 5 forums each day for a week, you could acquire a modest number of links and traffic before the week is out.

Or, using a semi-automated directory submitter you can probably grind through 100+ directories in that hour and then assuming an acceptance rate of about 60% would give you 60 extra links per day, not the best quality links though.  Directory links are usually not much good for traffic.

What if there aren't many active forums in my niche? I have found some, but they are not active. They are mostly very new or otherwise do not have many members or new posts.