imho there is no right way of determining the value of "good idea to reboot".
depending on your workflow the number of users is a interesting value. in other cases it's more a question of traffic.
i would begin:
isi statistics pstat --protocol=smb2
to see how many protocol traffic is going on, combined with Network Output / disk I/O i get a Feeling HOW MUCH the smb users are doing.
with isi smb sessions list
i can look WHO is doing stuff (maybe my CEO is reading his pdf-newspaper may be more important than a higher number of "normal folks")
with isi smb openfiles list
i can look WHAT these People are doing (are they all just reading a Manual or are they doing some DNS-sequencing)
with isi statistics client
i can look further in HOW the used tools are doing the workload. This would be quite uninteresting in case of maintenance and more something for workload / performance analytics.