They should send an email OR postal letter if FP are required.
FP request can be at any stage, a few weeks after IP or 15 months after you apply as one friend had.
Having provided them in the past should not make a difference as they're meant to wipe them our afterwards, that and IRCC isn't amazing at linking things (except of course how I know a load of people who've had their Citizenship application address refer to some ancient address from not even PR applications but old work permits).
It does not take a long time. I think I was at the center for giving mine less than 15 minutes (obviously doesn't help if you aren't near a center).