The game's milestones are generally based on story progress, not level.
Chapter 1 is shortly after Alderaan, as they have said, but that allows for a wide range of levels.
Starting planet -> Faction capital -> Nar Shaddaa -> Tattooine -> Alderaan, with whatever other story-based running around might be involved in there.
That is the minimum that you will do to complete chapter 1.
Add in PvP, flashpoints, and any non-class missions that you choose to do and nothing stops you from being level 50 before you finish chapter 1.