I play in a sportsbar for about 100 people nightly, mostly in their early 20s. I keep an MP3 player full of the latest top 40 pop, rap, and R&B that I refresh monthly. I play these songs as filler but will let it go depending on how the crowd reacts when the beat drops on each song. If there is a bigger rotation (15+), I'll usually just play until the next singer gets up, which seems to be about 30-45 seconds most of the time because it is a big bar and is almost always full.
I do save my best dance songs for when I have to use the restroom
and for the end of the night when even the karaoke peeps are toasty and seem to get more into the filler music!!
It is also key to play really upbeat filler after someone sings one of those wonderful balads that no one seems to like except the one who sang it (you all know the ones!!)
That's my 2c!!