How long did you need to hold down the power buttons? Literally minutes, or was it 1 min or 20 sec or something like that?

My Nexus 7 2012 sat unused for probably 6+ months. I replaced the motherboard following these directions, and have triple-checked that everything is connected correctly. The battery is getting charge--the wall plug and device feel warm when it's plugged in. However, unable to get any response out of the device. I've held all combinations of buttons (I seem to remember getting into boot mode with volume up + power) but nothing happens.