I need to experiment with it a little bit, but at level 8 I managed to get a proc after 30 uses of Flares. Not sure if it is similar to enhancement procs and it's more likely to fire with a high recharge power
Edit- RftL seems to proc reliably in my Rain of Fire power, which is at 90sec recharge (as long as it's not on icd)- so you can kinda predict it, but it isn't really an opener