Wandering patrols - how often?

[Halaster Blackcloak]

On the sub-level, which is a twisting maze of convoluted corridors, there are patrols of a specific type of monster. They are constantly patrolling, and there are 4 sets of monsters patrolling the area. This is the size of Level 1 of Undermountain, or Level 7 (see the preview pic on the preview thread).

So my question is, what would be a reasonably frequency for encountering such a patrol? I originally wrote a 60% chance per hour, but that's crazy. I was then thinking a 35% or 45% chance every 2 hours or maybe every 3 hours.

I also thought about a 45% chance of a patrol arriving in 2d10 rounds after any commotion is made.

Any thoughts? The monsters patrolling the area know every nook and cranny, so they don't waste time getting lost or losing their way. What would you consider deadly but still reasonably realistic?

[Tawnos76]

45% every 3 to 4 hours.
Treat it like a security job where you have the patrols every 3 hours or every 4 hours.