What is the normal memory usage?

You can check the memory utilization from the tmsh and verify the tmm memory usage. If it is more than 70% you might need to worry about really. As per the graph, you have enough tmm free memory i think.

Sys::System Memory Information
------------------------------------------------------------------
Memory Used(bytes)  Current  Average  Max(since 01/13/17 08:33:34)
------------------------------------------------------------------
Total Phys Memory     11.1G    11.1G                         11.1G
OS Used Memory        11.0G    11.0G                         11.1G
TMM Alloc Memory       5.1G     5.1G                          5.1G
TMM Used Memory      580.7M   569.5M                        584.2M
OS Used Swap              0        0                             0

You can read about the memory allocation here

-Jinshu