Adjust topic title length on topic listing page (Advanced Forum)
Not sure if this is exactly what you're looking for, but Advanced Forum integrates itself with Views.

So (assuming you have the Views module), you can trim the length of the title field in the advanced_forum_topic_list view. This c done by clicking the Content: Title (Topic / Topic starter) field, going to 'Rewrite results' and checking 'Trim this field to a maximum length'.

Because the field contains HTML to insert a span tag, you'll have to add 32 to the trim count (e.g. if you want to trim to 20 chars, enter 52), and ensure 'Field can contain HTML' is checked.