That depends on several factors:
- How many players there are
- How familiar with the game everyone is (less familiarity takes longer since people need more time to read the cards, debate what they mean, etc.)
- How fast your players are in general at taking their turn and deciding what to do
It's been my experience that a game of Epic Munchkin with 3 players takes about as long as a game of non-Epic Munchkin with 5. And your results may vary. But I'd give yourself at least 3 hours, with wiggle room for 4. :)