How long is an "average mission"? My mission (the first in my campaign) is around 7 minutes long and has 16 events, many of which have multiple sexps, and there are also other sexps for debriefings and arrival/departure cues. Basically it's like this:
You fly out of the Moloch's hangar bay and are greeted by six Anubises, one of whom taunts you. You blow them up.
An Anuket gas miner and three wings of three escort fighters warp in. You are ordered to destroy the Anuket and the fighters. The escort wings respawn three times. Then Sigma wing, composed of Dragons, warps in and relieves you of your watch (they're on your side so don't shoot them!). Mission accomplished.
Available loadout:
Ships: GTF Apollo, GTF Valkyrie, GVF Seth
Primaries: PLS-10 plasma cannon, Subach HL-7, VLL-9 Vasudan Light Laser, Disruptor, A-Disruptor, Akheton SDG, Prometheus R
Secondaries: MX-50, Rockeye, Fury, Fang, Tempest, Interceptor, Harpoon, Phoenix V, Hornet
Recommended loadout: GTF Apollo with a PLS-10 and a Subach, with two banks of Harpoons