The Reset time can be split into two parts.
The first part is when any of the Reset sources are active. This part depends on the input to the Reset sources. The external Reset is active as long as the RESET pin is low. The Power-on Reset (POR) and the Brown-out Detector (BOD) are active as long as the supply voltage is below the Reset source threshold.
The second part is when all the Reset sources are released, and an internal Reset initialization of the device is done. This time will be increased with the start-up time given by the Start-Up Time Setting (SUT) bit field in the System Configuration 1 (FUSE.SYSCFG1) fuse when the reset is caused by a Power Supply Reset Source. The internal Reset initialization time will also increase if the Cyclic Redundancy Check Memory Scan (CRCSCAN) is configured to run at start-up. This configuration can be changed in the CRC Source (CRCSRC) bit field in the System Configuration 0 (FUSE.SYSCFG0) fuse.