How long does it take an iPad Pro 1st generation to charge

I have two,1st gen iPad Pro models.

Using the supplied 12W adapter, from 5% to 100% takes approx 4-4-1/2 hours.

The 1st gen iPad Pro supports fast charging.

Using the newest Apple 20W USB-C adapter with an Apple or third party, high quality Mfi certified (Made for iPad, iPhone, iPod ) USB-C to Lightning cable, the 1st gen iPad Pro will charge under 3 hours ( anywhere between 2-1/2 to 2 hours, 45 minutes, or so).

Best of Luck to You!

Charge to what percentage?

Full charge takes over an hour.

The time required to charge your iPad will be determined by the initial state of battery charge, the power rating of the Power Adapter - and the specification of the cable that connects the two together.

The fastest charging rate will be achieved using a 18/20W Power Adapter that supports USB-PD (Power Delivery) over a USB-C cable.