The only good way to determine this for a given site is to operate all night for several seasons. Understandably, many people can not do this and they just do the best that they feel they can. In Maryland we seldom net over 10 owls a night. During 1993 Cape May had a night when 70 owls were netted. When this constant a movement is occurring constant net checking is a must.
Owls are very hardy and much less affected by stress than passerines and even some diurnal raptors.
However, their safety can be affected by predation and extreme weather such as wind, rain, snow, or even freezing temperatures that can freeze the toes of these otherwise well insulated owls. It is important to adjust to weather conditions and resolve any predator problems swiftly. A bander faced with an unexpected large flight or poor weather can shut off the lure, effectively stopping any further captures, and close nets as they are cleared until it is safe and responsible to continue netting. So that comparisons can be made between stations it is useful to record three important variables, hours of audio lure time, area of nets, and height of capture, for calculation of catch per effort. When similar sets of 6-7 double high nets are run it may be adequate to normalize as owls netted per hour of audio lure time.