How come it could be one translation with "ein" and one with "eine"? Isn't it suppose to be "eine" (because of the "Stunde")
Both are fine. It's common to omit the 'e' in expressions like this one but it's safe to just stick to 'eine' (which admittedly makes more sense). If you leave out the second part of the sentence "Es dauert eine Stunde" you have to use 'eine'.