bristlenose plecos and spawning

I have a pair of bristlenose plecos that have spawned twice. The first time was a complete surprise just for the fact that I had gotten them for the algae and had no idea how to even sex them. haha So needless to say I didnt know when she laid the eggs, and I didnt know when they hatched so the fry was eaten. Than the second batch came, again I dont know when, and I saw them hatch and removed the fry from the tank. My females belly is looking large again, and I was wondering how often do they spawn?