Should I charge my Apple watch every night?

The Apple Watch Series 3 should be charged each night, depending on usage. You may find it most convenient to charge your Apple Watch nightly, overnight, enabling you to start each day with a fully charged battery. The battery will not suffer any harm from regular charging.

How long does Apple Watch take to charge from deceased?

According to Apple, the Apple Watch Series 6 will take approximately one hour to charge from zero to 80%, and about 90 minutes to charge from zero to 100% charge.