The grace period is the first 90 days of mining once an M2 Pro has been registered and started mining.
Withdrawing MXC during your grace period does not affect earnings. However, it does affect your Miner Health.
This means that if you do not replace the fuel once your grace period ends, your earnings will be affected, starting the day after (i.e. day 91).
The grace period gives the community the chance to experiment with moving their MXC around without affecting earnings.