Someone on here completed the game at level 13. Most people will complete the game around 17-20 if they miss a bunch of stuff. 22 if they poke their nose into every nook and cranny.
It used to be lower than that, but some quests were not giving non-combat "Charisma" experience when they should have been in earlier patches.
You can expect to miss maybe 1 single level if you still check every nook but make choices that reward less experience.
