Hi,
The time is ~2.3s per LSB but the Sleep command uses the Watchdog Timer as it's timebase and it is driven by a RC network internal to the PIC. The value of this RC network varies from PIC to PIC and also with temperature.

If I read the datasheet correct (it's 07.00 in the morning here) the watchdog period is 18mS typical but can vary between 7 and 33mS worst case.

So, you just can't expect any acuarcy from the SLEEP or NAP commands.

Datasheet DS41190A
Section 9.6 and Table 12.4, Param.31

/Henrik Olsson.