As soon as you ovulate, pretty much! Although you may get normal cycles back straight away, it also may take a bit longer, so fingers crossed you're in the straight back to normal group! (BTW, not a silly question at all!)

ETA: My first cycle off the Pill was 32 days, then I had 4 cycles at 6 weeks-ish in length (which I accepted as being normal for me), then a really weird one at 8 weeks, towards the end of that one DH decided to ditch the condoms and I fell pg right after that.