A question that I was asking myself a lot during my playthrough.

There is 4 main area, here listed by expected completion order: Cyseal, Hiberheim, Luculla forest, and Phantom forest.

By time spent, Luculla forest > Cyseal > Phantom Forest > Hiberheim

A good indicator of your progression in the game is your level. If you follow the expected order, leveling up time should be linear. By doing 90% of the quest of the game and without exploit, the game should be over around level 21.