Unfortunately, it's a bit more complicated than the PLC5. Each task has it's own scan time. The monitor tab of the task properties page will have the information. You can get it progammatically by the GSV instruction as PeterW said.
However, interpreting what it means can be more complicated. For instance, if your main task reports a scan time of 20ms you may think that means it executes 50 times each second. Well, not quite. If you don't have much in the way of other tasks and communications, it will be very close to true. The communications and overhead shares the lowest priority with the main task. The more comms you do, the more impact. All other tasks are always higher priority and will interrupt the main task to execute. Obviously, the longer their scan times are and the more often they execute the more impact they have on the main task. You can see this difference on the main task property page. Interval time is the elapsed time between the main task getting to the top of the program.
The good news is that if you have a fast processor like a 1756-L6x the difference doesn't usually become a problem. I have a L61 here with a fairly large program in it with 3 periodic tasks (relatively small) executing at 100ms, 500ms and 1000ms. My max scan is 75.842000ms and my max interval is 75.886000ms. The only comms at the moment are to my RSlogix5000 pc. If you are using an older L1 processor or a CompactLogix, this can be a lot more dramatic.