SQL Memory Usage Cumulative for each Database
I don't believe there is any cumulative / average metric for this. Nor do I think it makes sense to look at it this way. I don't think you can derive any useful conclusion from such a view of things. This information is too volatile to use in that way. Some stuff ages out faster than others. It greatly depends on which pages are being accessed frequently, how much memory is available on the system, etc. Also, the Buffer Pool is not the only use of memory. There is also permissions cache, plan cache, DB Metadata cache, and others. Hence, such an approach will most likely be misleading.

In addition to looking in sys.dm_os_buffer_descriptors, there are also the following resources to check:


DBCC MEMORYSTATUS;
SELECT * FROM sys.dm_os_memory_clerks;
SELECT * FROM sys.dm_os_performance_counters;


Regarding those three resources, please note:


there is some duplication between the DBCC and the others (and even some other DMVs not listed here)
in each of those resources, there are some entries that cover all databases and cannot easily be broken down, such as Connection Pool cache, plan cache, etc