How long does one charge last?

The maker says the rechargeable system works for weeks after one charge through a USB connection. Actual runtime depends on how often and how long you grind.

Does it work on both salt and pepper?

Yes. GrindPep is sold as a salt and pepper grinder, and the ceramic mechanism handles both. Very coarse rock salt or oily spices may grind less evenly.

Can I change the grind size?

The product information does not describe a grind-size adjustment. It presents the ceramic mechanism as delivering uniform grinding rather than selectable coarseness.

Do I need to buy batteries?

No. It charges over USB, which removes the repeated cost of disposable batteries. You will need a USB charger or port.

Is it better than a manual mill?

It removes the twisting: press one button and the seasoning lands where you want it. A manual mill never needs charging, so the trade-off is convenience against a battery to top up.