There are two hidden defaults which control how often OmniFocus complains about stale clients:
SyncStaleClientInterval (factory setting 604800 seconds, or one week) says how stale a client has to get before OmniFocus complains the first time.
SyncStaleClientComplaintInterval (factory setting 172800 seconds, or two days) says how frequently to remind the user after the first complaint.
So if you want your first warning after two weeks (instead of one), and subsequent reminders every four weeks (instead of two days), try entering these commands in a Terminal window:
defaults write com.omnigroup.OmniFocus SyncStaleClientInterval -float 1209600
defaults write com.omnigroup.OmniFocus SyncStaleClientComplaintInterval -float 2419200
After you've done this, restart OmniFocus so it notices the new settings, and you'll be warned much less frequently!