How long does it take to transfer 88 GB of data from an old iPad to a new iPad Pro?

88GB is a lot of data...

The time required will be determined by many factors, including but not limited to:

• how much data must be restored
• the speed/throughput of your internet connection
• the speed/throughput of your WiFi network
• server loading; assuming an iCloud backup, Apple’s regional servers may be busy

The only aspect that you may be able to directly influence is your WiFi network. You should note, however, that the slowest item in the chain will ultimately be the limiting factor.

As rough guide, if transferring directly from one device to the other - establish the maximum throughput of your WiFi connection - and perform some relatively simple math.

Remember, to use the same units of measurement when performing the math; network throughput is generally expressed in megabits per second (Mb/s) - whereas you are quoting the quantity of data in gigabytes (GB). This web-page may help you with the math:

https://www.wikihow.com/Calculate-Data-Transfer-Rate#:~:text=Divide%2520the%2520amount%2520of%2520data,equation%2520T%2520%253D%2520A%2520%C3%B7%2520S.

For practical purposes, allowing for protocol overheads, anticipate achieving no more than half of the maximum theoretical throughput. As such, if you double the calculated time to transfer the data - you will have a reasonable approximation.