Usually it takes about 3 to 4 hours. Sometimes the tablet will charge in less than an hour for no reason. But as I said, it’s random.
But in both situations, if you charge the Nexus 7 from a normal power supply, not from a PC or laptop by using an USB cable.
For old tablets and since the Nexus 2012 is pretty old by now, (2015), the battery might be so used that it will take even longer than 3-4 hours to charge it. Sometimes when you charge it after 3-4 hours, the tablet will display a 15-20% energy level In that case, keep it connected to the power source, open it (keep it open) and check the charging value from time to time.
If you haven’t used your tablet for days/weeks, when you are trying to charge it, it will stay black for even one hour until it has enough power to start.
In case that your Nexus 7 is displaying let’s say 30-40% charged, put it on battery safe on. This option is available since Android 5.0. It will consume less energy but you will only be able to watch YouTube or use Chrome, in good conditions.