Perhaps the best way to determine the average size of a screen recording is to measure it. Simply make a screen recording for one minute - end the recording - then check the file size. This will provide an approximate value that is good for your model of iPad.
Now that you have established the storage requirements for one minute of screen recording, you can divide your available/spare storage by this value to give you your maximum recording time. You should aim to keep at least 5GB of free storage - as your iPad needs some space to work properly.
If your internal storage is full, your only option is to “manage” the storage that you have - and delete unused Apps or unneeded data.
These support pages should prove to be helpful:
About storage on your device and in iCloud – Apple Support
What's the difference between device storage and iCloud storage? – Apple Support
Manage your iCloud storage – Apple Support
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
Clear Other storage on your iPhone or iPad - Apple Support