The Reset time can be split in two.
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 Brown-out Detector (BOD) is active as long as the supply voltage is below Reset source threshold.
When all the Reset sources are released, 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 fuse setting (SUT in FUSE.SYSCFG1). The internal Reset initialization time will also increase if the CRC source is set up to run (CRCSRC in FUSE.SYSCFG0).