Always on. It's not much of a battery drain risk unless you leave it plugged in for an extended period with the key off. As noted above, it's really handy to be able to leave things plugged in while the bike isn't running. I've run my GPS for years this way, and haven't had any issues.
Be very careful with the charger in damp conditions. The plug that goes into your tablet is likely a micro USB. They are not noted for their robust nature, and there are numerous reports of folks frying stuff by plugging in a wet cable.
Originally Posted by S H I F T Y
I was just going to use the car charger that came with it not a usb. So if it is hooked to the battery, is it always on, or just on if the key is on?