Apple Watch time

What to you mean by "dead". If the watch is dead meaning it has run out of battery, it doesn't show anything.

If you mean how long can it continue to display the time in power reserve mode, I believe it's up to 72 hours.

Check your battery and charge your Apple Watch - Apple Support

15 or 70 seconds

If the watch is dead (battery discharged or fully dead watch) it has a blank screen and does not know what time it is because the electronics stopped working.

No still can’t guess what you are asking. The Watch will last up to 18 hours before the battery dies