Sadly, it's pretty short (and easy once you figure out the killer combo for each faction).
For the record, each faction has a unit that gets a second upgrade. Infantry for Westeria, Gunners for the Cult and Cavalry for the Horde. Spam the hell out of those. If you start without the building you need for that, make getting one your first priority.
Cult mechs in particular completely destroy everything else in the game. Ogres (and infantry units) die without landing a single hit, hostile ranged units take off at best 1/10 hp, cavalry occasionally manages to get your mechs down to 2/3.
For Westeria it's a little trickier. Their 'super' unit is extremely tough but takes a long time to kill anything, so by the time you've finished off one defender in a stronghold you won't have time for more than one or two hits on the building itself before the next enemy unit spawns. The trick is to use your banners whenever possible and add a single unit of monks/zealots to your attacking force for the morale bonus.
The Horde's main weakness is that you can only recruit their elite unit in castles, and both their town units are kinda lackluster. They do get the most powerful spell, though. Building up your mana is a priority, since pretty much nothing can defend against even your below-average infantry when two worms are used in a row.