How to change max_length of Django's username field

Question

I'm using the default Django User backend. I just want to make one small change: reduce the max_length of username to 30 (I believe the default is 150). How would I achieve this?

I'm using django-allauth as my user account framework and here is my current custom allauth signup form:

draft1/forms

class AllauthSignupForm(forms.Form):

    captcha = ReCaptchaField(
        public_key=config("RECAPTCHA_PUBLIC_KEY"),
        private_key=config("RECAPTCHA_PRIVATE_KEY"),
    )

    class Meta:
        model = User 

    def signup(self, request, user):
        """ Required, or else it throws deprecation warnings """
        pass

draft1/settings.py

ACCOUNT_SIGNUP_FORM_CLASS = 'draft1.forms.AllauthSignupForm'

Any idea how I can change the username max_length to 30?