Can we use ipad air while charging for a long time?

Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. 

When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.

Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

You can use it while charging but it is not recommended to do so for the charging process to be at most optimal