How Long On Average Does It Take To Clean An IP Camera?

On a bench or installed in the field?

15 minutes to 30 minutes assuming an average sized commercial site. 

Most of your time will be moving from camera to camera. If it is a large plant or you need booms/lifts it can add significant amount of time.

I've done this and while some cameras are super quick to do, meaning around 3 mins or so.  While others are more difficult due to location, etc. and I rarely remove the dome cover to directly clean the lens.  I only do this in locations like the kitchen of a restaurant and if so, I wash the dome cover with soap and water to get the grease off.  The cameras that are higher really only get their dome covers clean because they are usually pretty air-tight so very little dust gets in.