Not long really. I've played plenty of strategy games where the entire game takes ages (any of the total war games, or Empire at War for example), but the discrete battles themselves usually are rather brief.
However, there have been games in Dawn of War 2 in a 3 v 3 where neither time can seize the second point for any length of time and thus the game takes ages to finish. The worst case has a particular point mere moments from either team's base so capturing or holding it is a slugging match. In one example, I was playing as a Techmarine with a chaos and eldar buddy against 3(!) Tyranid armies. Inevitably, two armies would try and cap while one was off rebuilding their forces. Thanks to the sheer weight in numbers plus lots of those pain in the ass Carnifexes, any attempt to cap was met with heavy casualties and often didn't result in the point changing hands at all. It was then left to a single army to hold back the weak counter attack. Then there was a brief pause while the cycle repeated itself.
Eventually, the lethality of the battlefield proved sufficient that the only things I could keep alive for any length of time (while still being useful) was a terminator squad, a venerable dreadnought, my Techmarine, a pair of lascannon wielding devestator squads and my predator and a few turrets (that absorbed much of the punishment). Luckily, when the other team is too stupid to flank, this is sufficient firepower to turn almost any assault in short order, even if the odds were absolutely ludicrous (the above against a dozen gaut squads (8 units each), 3 - 6 carnifexes, 3 - 6 warrior squads, and in this case 3 hive tyrants). Thanks to the simple fact that they never once tried to sneak in to disrupt the lascannons/tank (which would mean their Carnifexes could just walk right over everything else), or never once tried to sneak in and bring down the turrets (thus ensuring most of the gauts and warriors would make it into range before being killed) or to simply lead their assault with the carnifexes to draw all the fire (which would lead to several dead carnifexes but a disrupted defense in short order) and instead just blobbed at me with 2 armies and then with one I was able to hold them with the previously mentioned army along with several eldar platforms (shiruken cannons, brightlance and a pair of d-cannons) in a terrible stalemate (my termis could generally get to the point and turn it to neutral but never could seize it before a new blob arrived) while the chaos player and the remainder of the eldar team wrecked shop elsewhere.
In the end, the game took an hour and 20 minutes (because the nid players kept sending out small task forces to counter our own efforts while blobbing the center). This is significant because the average 3v3 game takes, perhaps, 25 minutes to complete (and, barring a surrender, at least 10 minutes). That game netted me the highest score I ever got in a game (~2300) and the end totals demonstrate just how stupid the nids were being (I lost a total of 23 units (actually pretty staggering considering the cost of marine replacements) but KILLED 14 commanders and 270 units total!).
