How long does it take restore iphone while connecting to computer

Depends on how much data you are restoring. How long has it been?

GB

It usually take less than 15minutes to Restore iOS after iOS Downloaded & also if you want to Restore a Backup it may take Up Considerable time depends on your Backup Size !

In my own case restores take approximately 30 minutes including downloading the iOS. IMO anything between 15 - 60 minutes would be "typical".