While 行きます means just go somewhere, 行ってきます means the same but also implies to come back. The reason why it means not only go but also come back is that 行ってくる consists of 行く(go)＋くる (戻って来る come back.) However, it doesn't mean promising to come back. Even though the term of this expression is implying to come back as well, a lot of people use this as just go. If I say I go to the library, I am just explaining to go there, on the other hand, if I say "libraryに行ってきます," This also means 行く＋来る（戻ってくる）go to the library and will come back to here.

How long you'll be gone for doesn't matter when you use ～てくる form, as long as you come back eventually. The more I try to think of a difference between 行きます and 行ってきます, the less I come up with. Both mean "go" and can be translated "I will go to America," but アメリカに行きます sounds like a determination, and アメリカに行ってきます has a slight feeling of adventure (for good or bad) I think. You can use 行きます under any circumstances. 私は今週の土曜日、仕事に行く/行きます = I go to work this Saturday. 彼は今年の夏、アメリカに行きます = He goes to America this summer. 彼女は昨年、北海道へ行きました = She went to Hokkaido last year.