BHBF is 4 missions, 3 of which you have to unlock. The 2nd mission/3rd mission unlock depending on what you do in the first mission. Mission 1 can take...30-45 minutes depending on what you want to do, Mission 2 can take longer, Mission 3...probably about the same.
Mission 4 is basically the reward Mission for getting A on everything else. If you want an A-rank though, you're going to have to do a lot of saving/reloading because your troops accuracy is terrible.
Also keep in mind that to play the second mission, you have to do the first mission over again, so...yeah, it'd probably be a good idea to go for A rank on mission 1 the first time out.