From what documentation I received from Minelab:
USB charger with a minimum capacity of 2A @ +5V will give you a full charge in less than 4 hours..
An accessory USB charger (plugs into car outlets for example) with a minimum capacity of 2A @ +5V will give you a full charge in less than 4 hours..
A standard USB2 power on outlet on your PC / Laptop usually only has a max capacity of 0.5A @ +5V will give you a full charge in roughly 18 hours... (18!)
Do not use a mid capacity USB charger, for example, using a reputable and certified USB charger (Apple or Samsung) rated at 1A #+5V the charger will typically engage it's thermal protection safety functions and automatically shut down... the MD may NOT fully charge.
Personally, I use an in house USB charger of 2A and in car USB charger plug at the same capacity. I plug the MD in the car at the end of the day and it's usually recharged by the time I get home (depending how far I venture that day)..
Cheers