Hopefully the SS gurus will have more detailed and accurate information (undocumented "features" are among SH4's "endearing" traits), but in the stock 1.4/1.5 game, I use a 55 mile radius and plan to spend 48 hours in "the circle".
Leaving the patrol area seems to trigger a clock reset, as I've gone after convoys outside the patrol area, returned and it took a LONNNNG time to complete the mission...
