What level will I be on Borderlands 2 after completing the main game and all DLC?

On your first playthrough all enemies (except raid bosses) are capped at level 30. It's very hard to level up past about level 35 killing them because by then you'll be getting 1xp per kill.

On your second playthrough the enemies re-scale and level you up to 50, which is the current cap.

If you go through playthrough one you should be about level 35 by the end. None of the DLC will take you past that, because the enemies will still be capped at 30.

If you want to get to level 50 start TVHM mode, you will still be able to start the DLC right from the beginning.

Note that you can always switch back (via the character select menu) to playthrough one, and you keep all your kit between both, so there's really no excuse not to go to TVHM