It depends.
If you charge iPad from original 12 volts it would take 2 hours
If you charge iPad from iPhone "5 volts " or adopter or power bank would take 5 hours
If you charge iPad from Bluetooth speaker that is 40 watts like Xb41 or Xtreme 2 or
studio 5, boombox would take 7 hours
Now if you try to charge your iPad VIA iPad Pro 2018, Laptop, and desktop would drain it first unless it is charged. Assuming those devices were plugged in would take at least 10 to 12 hours