Here's what B&N says about charging the HD+
It will take about 4-5 hours to fully charge your NOOK using the Power Kit plugged into a wall outlet. Do NOT turn off your NOOK while it is charging; just allow it to go into sleep mode automatically.
Mine usually charges in the range of 4-5 hours, but typically I plug it in at night and it's fully charged the next morning. I cut the Wi-Fi off while charging and use the adapter that came with the Nook.