if you are talking about housing benefit i think the actual term is "regular" and this is open to interpretation, depending on where you live ... I once rang up the housing benefit office in berkshire on behalf of a client of mine and they said their interpretation of "regular" could be even 1 night a week. They also told me that if the person declares a regular overnight stay, it didnt mean they lost the benefit completely, just that the person staying would be asked to contribute a percentage (ie if they stayed 1 might then it would be 1/7th of the housing benefit).
Your best bet would be to phone your local office yourself and discuss the situation - after all if you are making a general enquiry, no need to state your name