I have a similar situation and have been fooled twice by the same trap this year. :(
Lots of activity, similar to Capt's pic, for multiple days...I'd say up to a week. The only thing in the Nuc is a piece of old comb and some lemon grass oil. On both occasions I would have swore bees had moved in, yet I seal it up at night, take it down, and it's empty. :laugh:
Both times when I took it down after dark I was suspicious because I saw no bees at the entrance, however, It's a double stacked 6 frame Nuc and I "assumed" they were just clustered up top....They WON'T fool me again!