Dwarf vs. Wingayel (Lands of Outcasts)

If you're talking about the "Land of Outcasts" map, you can easily reach level 20+ within the first month (with any race, any hero). In the second month, you can gain another 5-6 levels + 4 Silanna trees, resulting in a level 30+ hero. In the third and fourth months, you can run through the utopias again (for artifacts and magic), but it becomes difficult to gain levels because you've run out of neutral creatures.
On generated maps, gaining experience is easier (as Ereb correctly pointed out), and you don't necessarily have to spend 3-4 months waiting to reach level 19.