If she genuinely regrets it then you are quite lucky.

Marriage is a marathon and a couple of years for her hair to grow out to BSL isn't too bad even though it seems grim at the moment.

The urge to chop long hair after getting married is strong for lots of reasons. She may also be testing you. Best case is that she actually wanted a change on a whim and she actually regrets chopping so much.

Hopefully over time you can have open level-headed conversations about each other's preferences, and how you can please each other.