As long as the batteries in your UV flashlight last!

We only kept ours for the one play session, but I’d expect them to last a few days, especially if you store them in an air-tight container between plays. After that I’d suggest making new ones because:
a) the water beads may shrink (as the water component of the tonic water starts to evaporate)
b) they might attract ants (especially if you used tonic that contains sugar)
and c) bacteria introduced from kids hands might make them ‘go off’.