yes it is. I'd budgeted for 4-5 months of time, figuring it might go to 5-6. I'm in month 9, and it will be at least another month. :/ Huge lesson learned, and hopefully this will be the last time I learn it. I don't typically take on projects this large, and in the past if there was a 80% time estimation error, the impact wouldn't have been as dramatic. :/
In my experience there's one thing you need to watch out for, and that's being a nice guy. You might start feeling sorry for your client because stuff is taking longer than planned, which makes it harder to say no to just minor changes to the scope. They will only take you 5 minutes each, and it keeps the client of your back. Those are the things that really get to you when it comes to staying motivated I think.