From personal experience, I know I can turn out three missions that, if not masterpieces of quality, at least didn't have anyone screaming "NOOOOOBBBBB", very quickly. Those were the first three missions of Operation Savior, completed in roughly eight hours. Then I hit bugs with the build in use at the time that delayed the campaign for a week. Had I not, I probably would have made it to the "five-missions 24-hours" deadline I originally set for myself on the campaign with time to spare.
Admittedly, the campaign included no mods, and no voiceacting. It was not as basic as it looked, since I went to some length to try and cover multiple contingencies regarding ships being disabled and destroyed in the last two missions (and they consequently took about half as long again each). But I think this should be lesson of sorts. More people should be trying to emulate Homesick (which was also no-mods no-voice), and fewer people trying to emulate TBP (total conversion) or Inferno (total conversion in all but name).