I'd assume standard toggle rules would apply.
A chance when you first activate the power and then every 10 seconds thereafter that the toggle is running.
as a Bubbler I'd stick it in Repulsion Bomb. With a base recharge of 30 seconds it's going to proc very often when using that.
That being said, this IO has always seems of dubious use to me. If a power isn't "recharged" within its 5 second window it seems to reset back to where it was before the proc, minus 5 seconds. Making the whole thing mostly futile unless you're using it for shorter recharge powers like attacks, or luck out on your long-recharge powers timing wise.
So I'd imagine its great in Super Strength etc, not so great for recharging multi-minute powers though.
I've certainly not noticed this when I've used it - AFAIK recharge works so that if you have the proc for 5 seconds, you just get extra recharge "ticks" for those seconds. I've been using it to get to permadom quicker. I could be wrong, but otherwise what you are saying seems to suggest it would take longer for long recharge powers to recharge which doesn't make sense. I've used it a lot on long recharge powers and it seems to work well.