In Django/python, how do I set the memcache to infinite time?

Support for non-expiring cache has been added in Django 1.6 by setting timeout=None

def _get_memcache_timeout(self, timeout):
    """
    Memcached deals with long (> 30 days) timeouts in a special
    way. Call this function to obtain a safe value for your timeout.
    """
    timeout = timeout or self.default_timeout
    if timeout > 2592000: # 60*60*24*30, 30 days
        # See http://code.google.com/p/memcached/wiki/FAQ
        # "You can set expire times up to 30 days in the future. After that
        # memcached interprets it as a date, and will expire the item after
        # said date. This is a simple (but obscure) mechanic."
        #
        # This means that we have to switch to absolute timestamps.
        timeout += int(time.time())
    return timeout

And from the FAQ:

What are the limits on setting expire time? (why is there a 30 day limit?)

You can set expire times up to 30 days in the future. After that memcached interprets it as a date, and will expire the item after said date. This is a simple (but obscure) mechanic.

From the docs:

If the value of this settings is None, cache entries will not expire.

Notably, this is different from how the expiration time works in the Memcache standard protocol:

Expiration times can be set from 0, meaning "never expire", to 30 days. Any time higher than 30 days is interpreted as a unix timestamp date

So, to set a key to never expire, set the timeout to None if you're using Django's cache abstraction, or 0 if you're using Memcache more directly.