Ubisoft forum manager bukowski113 posted the details on the game's official forum:
You have 5 activations on 3 separate PCs. Uninstalling the game "refunds" an activation. This process is called "revoke", so as long as you complete proper uninstall you will be able to install the game an unlimited number of times on 3 systems. You can upgrade your computer as many time as you want (using our revoke system) Ubisoft is committed to the support of our games, and additional activations can be provided. Ubisoft is committed to the long term support of our games: you'll always be able to play Far Cry 2.
EA CEO John Ricitiello recently spoke out on the subject, admitting that while he doesn't personally like DRM, it is needed to fight piracy and is "something that 99.8 percent of users wouldn't notice."