simple billing strategy

models.py

import arrow

class Org(models.Model):
  last_bill = models.DateTimeField(auto_now_add=True)


class User(models.Model):
  org = models.ForeignKey('Org')
  last_login = models.DateTimeField() # needs to be updated as part of login process

tasks.py

import arrow


@app.task
def run_billing():
  now = arrow.utcnow()
  orgs_to_bill = Org.objects.filter(last_bill__lte=now.replace(days=-30).datetime)
  for org in orgs_to_bill:
    users = Users.objects.filter(org=org, last_login__gte=org.last_bill)
    # generate invoice based on len(users)
    org.last_bill = now.datetime
    org.save()