Totally agree with the above statement, but want to add that it really depends on what you are trying to accomplish. For my own part, when I create missions, I'm looking to recreate the feel of a specific event, or day. You gather a few of these, and you can recreate a time frame reference, a general feel of a specific time, in history, or in the story you want to tell. A few of these blocks of time is plenty to call a campaign. IMO, a campaign is the story leading you towards the conclusion, it doesn't have to be a very long string of doing the same few things over and over if the story is told well.
I think a good example might be FlatSpinMan's Legion Kondor SCW campaign. It certainly wasn't big, with a few missions in the bi-planes as training, a flight in a Tante Ju, then more in the He-53 (?) til you moved to the 109s. I believe it was less than 35 missions, but it really really told a longer story than some other campaigns I could name. You just need a taste of each phase you want the user to experience, less work, and more impact without becoming repetitive.