Every show that I have worked on has had overtime--it is a reality of VFX post production. Working 80 hours a week for over a month or two is typical. I have done a 120 hour week one time--boy that was crazy.
This is how the overtime typically works. Early in a production there will be no overtime. When things start picking up they will usually tell you that you can work up to a certain amount of hours, like 60 or something, but they leave it up to you. Then the 60 will be mandatory, then 70, then 80 and so on. You will go to a 6 day work week (working Saturdays) and then if things are really crazy you will go to a full 7 day work week. However, I have only worked a couple of Sundays, preferring to get my time in throughout the week.