This means that the only factor that changes are the passengers on board.
We always make sure to scan the cabin for potential suitors, it is very easily done during the in-flight service and comes as second nature.
So to answer how often, then I would say at least once per day! Another thing is if we actually act on it. That always depends on the passenger, if he/she is travelling alone, if they make eye contact with us, interact more than usual with us etc…
An obvious sign that there is mutual interest is if a passenger gets out of there seat to come have a chat in the galley with us.