how long does it take to update software

It is impossible to give a definitive/absolute answer.

The time required to update your iPad will be significantly influenced by a number of factors. These include the speed and throughput of your WiFi network and internet connection, the amount of available unused storage space in your iPad, and the Update Server load.

With good network and internet connectivity, many updates can be completed in a half-hour or so. Some updates may take longer, possibly an hour or more.