For those who work by shifts, like those who do reactive support, there may be a requirement to work on call for a certain amount of time per month. However, just as equally there may be no requirement to work overtime/off shift since there's another group of people coming in after you to keep working.
For those who do proactive work, like administrators, operators, and architects, there is usually the expectation that if something breaks you will be available to fix it. There may also be the expectation that when needed you could work overnight or over the weekend to implement changes to the computer systems you're managing.
I personally do proactive support, like helping customers with design and optimizing their networks. Most of the time I don't work over 40 hours a week, but if something breaks I may work up to 50 hours that week.
The IT field can be very diverse depending on the type of work you do. If you'll be responsible for fixing issues/outages, there is a strong possibility that overtime would be needed as you might be the person responsible for seeing the fix through to resolution. You may be expected to be in charge of running conference calls that will discuss the issue/outage and what's being done to fix it. These calls may be with other IT folks as well as customers or management, so along with having a strong technical background it's also good to be very organized if you're working in the IT industry.