it depends on the PL played and the number of ships - players who use ancients or fleets with a few large models will usually play faster than players who bring the swarm fleets that consists of large numbers of ships.
In the KOTJ tourneys I ran We used 3 different PL matches; skirmish usually ran short and finished more quickly than the raid, which played quicker than the battle games, which usually overran the 2+ hours.
Generally, I think 30 minutes + 30 minutes per PL is a decent thumbrule;
Skirmish 1.5 hours, raid, 2 hours, batttle 2.5 hours, war, maybe 3 hours, armageddon...maybe 3.5 hours.
Chernobyl