The official charge time for your Scroll is 16 hours. However, in practice I have found that my Scroll fully charges in around 10 hours from empty.
You can also charge your Scroll via the usb port with either a laptop/pc or a mobile phone charger with the correct connection. I use a Tomtom GPS charger. It should be noted that the usb charge time is considerably longer than when using the standard charger.
LiOn batteries such as the one fitted to the Scroll do not suffer from "memory effect" unlike the older NiMh batteries, but there is some evidence that LiOn batteries dont like being fully charged all the time.
Your Scroll will not over-charge if you leave it connected permanently, but the charger has a built-in thermal cut-out. If it gets too hot it will cease to function until it is allowed to cool.