Your question is ambiguous. Are you enquiring as to the potential lifespan of the battery - or the time that the iPad can operate before requiring recharge?
Apple quote the iPad’s typical battery lifespan in terms of remaining battery capacity, when compared with that when new, after a specific number of full charge cycles. One full charge cycle equates to any number of charging cycles that together sum to 100% (e.g., 40% + 50% + 10% = 100%).
The quoted battery lifespan for an iPad is 80% of original capacity after 1000 full recharge cycles. By contrast, iPhone (with much smaller batteries) has a quoted battery lifespan of 80% after 500 full charge cycles.
As for how long your iPad will run on a full charge, this is heavily dependent upon your usage…
High screen brightness and CPU/GPU intensive Apps will increase battery utilisation, reducing battery life before a recharge is necessary. As an estimate, whilst subjective in measurement - for moderate screen brightness and CPU load, 8-10% battery utilisation per hour is generally considered to be normal.
As the battery ages and capacity reduces, the achievable time between recharges will reduce. Whilst actual battery utilisation for the same load will be the same, the relative battery utilisation will seemingly increase. When the iPad battery is approaching the end of its serviceable lifespan, the relative battery utilisation may be 15-20% per hour.
