You should plan on that amount of time, but there's a good chance it won't take that long. The last couple of updates haven't taken as long as some before them. But there have been some that did take in the two hour or even longer range.
Also in the past some people experienced odd issues, and some even had to have the firmware update pushed to them again, and go through the process again if they "interrupted" the process at all while it was happening by doing anything to interact with the car, like opening doors, etc. Obviously this shouldn't happen, but it has happened. So my advice to you would be that once you do start the countdown timer to the update, leave the car (efore the update actually begins) and don't check it again for a couple of hours, and then check only from the outside, by looking in, to see if the firmware update is complete. (You'll see a message on the 17" display.)
These precautions are probably unnecessary, but my feeling is better safe than sorry.