Free Ubuntu One accounts start off with a generous 5 GB of online storage and now you earn even more storage for free! Ubuntu One has started a new referrals program that lets you and the people you invite each earn an extra 500 MB of storage when they sign up.
The referral link is located on the right side of the Dashboard when you log in via the website (outlined in red above).
There is no time limit on the storage you earn, it is yours to keep for as long as you have the account. The only limitation on the referral program is that you can invite up to a maximum of 40 people.