@NOCARRIER Two things to consider, though i have no specific math.. Cache, this is the largest but can be cleared if you dont wish to keep a history of foreign posts/media.. then local data.
The cache is based on the total number of folloers your system has, pretty much a backup of anything that comes through the federated timeline.
The local data itself tends to be small and managable except for maybe videos people uploaded, but that can be limited.
It basically just boils down to what policy you wish to have regarding the cache.