I just released an implementation of DurationField for Django to PyPi.
After a discussion with a few Django core developers, it seems like keeping the DurationField implementation as a separate, reusable application is the preferred option. By staying independent, we keep the ability to make changes independent of the Django release cycle. Additionally, we avoid adding bloat to Django-core. Since not all databases implement an interval or duration data type (PostgreSQL does), django-durationfield is in some ways a hack by using a bigint datatype to store integer instances of timedelta.