Well, the starting Terran Campaign actually has a 30 minute stage. But it flies by like nothing de to constant Zerg attacks. Each attack force being bigger than the last. For that Level, I found blocking off the roads with Bunkers full of four Infantry each do the job well.
All you have to remember is to repair the Bunkers damaged after each onslaught.
I don't know if there are any other levels like that one.
But otherwise, I think all of the levels are fairly short. When playing as the Zerg however, the biggest thing you have to worry about are those big ass Siege Tanks the Terran have. The Protoss are fairly easy, even with their Energy Shielding.
Yes, you hit a PC game I have knowledge of somewhat, which has no real difference from the Playstation or N64 versions. xD
You should at the most 6-10 Drones or whatever are used, to mine Minerals, and at least 3-6 mining Gases.
Hydralisks are the only creatures you really have to look out for when going against the Zerg with the Protoss or the Terran. Otherwise they are all fairly weak, even the flyers.
As the terran, your best bet is to combine Firebats with the Marines, and Snipers when you are able to get them.
I don't have a lot of experience with the Protoss, except that Dragoons are well adapted at fighting just about anything, they will be good attack forces in large groups.
That is all I have at the moment. ;3