I am trying to build an application which tries to find out if a system is suffering from backlog (# of requests which are open are more than what the team can handle)

I have the below parameters alone and with these how can I find the ‘ideal’ number of open requests so that the system does not have any backlog.

Priority    # of Requests / Month   # of days to resolve
--------    --------------------    --------------------
P-1         45                      1
P-2         90                      2
P-3         90                      3
P-4         585                     5
P-5         90                      10
--------    --------------------    --------------------
Total       900
--------    --------------------    --------------------

Since each request has its own ‘time to resolve’, I cannot do 900/21 (# of working days in a month) = 42.85.

Any idea/advice?