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.
Overtime majorly disrupts your life. You are stressed all the time and it wares on your spouse because you are not helping with the kids and the house. Your spouse misses you and your kids forget who you are. Despite all of that, my wife and I have tried to adapt. When we know I am going into a long stretch of overtime, my wife and kids will fly out to stay with family for a vacation out of state. This way they can be enjoying themselves while I have my nose to the grind stone. This does not work out all the time but we have done it a bunch.
On the flip side, overtime is a great opportunity to make some extra money. Because of overtime we have paid off our debts and have some savings. There are not too many jobs that you can have where you can make that money in such a short amount of time. So as painful as it is, it does pay off.
Depending where you work the structure overtime pays differently. Some places will only do overtime pay if it is over a certain number like 60 hours. Other places overtime pay is only for weekends and extra time worked throughout the week goes to vacation days. If you do not use those vacation days then it gets paid out at the end of your contract. Some places do time and a half, others only pay your regular hourly rate for overtime. As I have added things up, overtime pay in the US is much better then anywhere else. Even if you get paid a lower hourly rate at the end of a show you will most likely come out ahead of other places. When you are talking contracts with a company they will go over all that with you.