Hi,
It's not clear to me what FLASH you are talking about. Is it the microcontroller internal one, or any external chip. What exact type.
So we have to guess.
For external FLASH the erase is started with a command. Sending this command just takes a few microseconds.
No processing power is necessary after this ... for the erase. The FLASH will perform the erase ... even when the microcontroller is stopped.
Now it depends on how you know when the erase cycle has finished.
Either by reading a FLASH busy flag or by using a timeout.
During the erase cycle no FLASH access is allowed, except reading the busy flag. One software flag can do this.
Klaus