How long it will take to update iOS via PC manually?

Ok i got iphone 4s with iOS 5.1.1 i wanna update it to iOS 6.1.3 i just wanna know how much time it will take to update from 5.5.1.to 6.1.3 though manual way i mean download the file in PC and update?

Reason iim asking this i here i got frequent power shut down problem so i don't wanna mess up with the update so please let me know how long it will take to update?

Depends on your internet connection as you have to download the firmware. Personally I wouldn't update on that computer due to the problem you stated which could brick your device. If you could access another computer & do it from there.