Should I avoid using Rails cache for geocoding data without precautions/expiration (Rails4, geocoder)

As a newbie, I'm wondering if it's OK to keep geocoding data data in Rails Cache. Indeed I'd like it to hold for 1 million users for example, and it feels like what I'm doing could allow a sort of cache-explosion:)

Here's how i do it today: I'm keeping the geocoding data I get from geocoder gem: as follows:

Geocoder.configure({  
    ip_lookup: :maxmind,
    :maxmind => { :service => :country },
    :api_key => ENV['APIKEY'],
    :use_https => true,
    timeout: 3,
    # here it is: I use cache in order not to send every page load a request to maxmind
    cache:  Rails.cache
  })

In that configuration, how long will Rails keep all the geocoding data in the cache? Shouldn't I set sth like an expiration on each data cached? how long?

thanks, Mathieu