Well, I usually don't change my meta descriptions for my forum too often. I'll admit, I've had the same one for about 6 months now, and I may change it soon to have a better improvement that what is it now. How about you though? Do you even bother with your forum's meta description or are you more focused on the content of the page?
Once you have something you like, there's no reason to change it (unless you are changing it to reflect changes to your site, so now it's different). Tweaking it to improve it is obviously a good idea, but no need to change it just for sake of changing it.
It's one of those set it and forget it things for me. When I moved to XF back in March my meta keywords had been the same since 2007ish. IMHO, Google doesn't give them much weight any more because it's so easy to game. I don't worry much about them, TBH.
I am changing my meta descriptions for my pages, but not too often, only if I see that I have too less visits for that pages from Google. In this case I change that meta description for that pages, to see if something changes to better or not. Usually it helps ! And I always take care to have different meta descriptions for every of my pages, even if it is the almost same page - I mean it is a very little difference about that 2 pages, for example page 2, page 3 and so like these - with the same content and same title; and of course I modify also the titles a little, to not be the same.
I've actually changed/tweaked mine before to gain better search engine ranking. I think on xenforo the default description is only going to show on few pages anyway, other pages should show the first part of the thread. eg this page is Code: <meta name="description" content="Well, I usually don't change my meta descriptions for my forum too often. I'll admit, I've had the same one for about 6 months now, and I may change it..." /> It's something I do from time to time, it's interesting seeing what one word can do but it's also easy to drop rank completely if you mess up.