I know it’s a tad tricky to answer but there must be a rule of thumb somewhere?
There is no rule of thumb that I know of. There are too many variables to say with certainty that a WPMU + BP install needs “X MB” of DB space. It depends on the number of users, the activity level of each user, the overall site activity, whether you give them each a blog (or more), and what plugins you have installed and activated.
But, I suspect that having a DB size limit of 150 MB is a stretch just on a low-trafficked WPMU site. Add in BuddyPress and 100 active users, and you’ll quickly shoot through that limit.
As an example, on one of my WPMU + BP test platforms, I have 5 users and only two MU blogs with very few test blog posts. Since it is a test install only used by myself, it has very little activity thus very little data written to the DB.
The DB size of that simple setup is 1.8 MB. Imagine an active site with many blogs and 100 users. Your host’s DB limit of 150 MB would be quickly reached.
I have ~250 users and quite a few blogs, and my database is still below 50 mb. We’ve only been active for a few weeks though. Extrapolating wildly, I’d say a reasonably active 100 user installation would take a month to build up 50 mb and that is three months to build up to your limit, 150 mb.
So in the short run, it is sufficient, but after a while you’ll have to move or sweep out old stuff from your database.
Thanks guys, both replies were helpful.