Submitting to sitemap to yahoo

how often is recommended ?

It of course will vary on the popularity of your forums, there is no point in daily submissions if you are only attaining a small number of new threads. But if for example you are gaining 100+ pages of new threads then it might be viable to allow Yahoo! to keep upto date with all of these new additions.

I personally submit around every week or so.
No need to overload them