Tickets are valid for two days but also cover the Colosseum and Palatine Hill. That's a lot to see so make sure you arrive there nice and early on both days! The Forum opens from 8:30AM (except holidays), but closing times vary depending on the season. You'll have more time between April and September.
If not booking in advance, use the Palatine Hill ticket office, as the queue will be mercifully short compared to that at the Colosseum!