How does Rammus's Q speedup work?

When Rammus presses Q, he gains a speed boost that scales up by 20% per second, for an eventual maximum of 140% at the end of the duration.

The wiki page will serve you far better than Google in almost all cases.

http://leagueoflegends.wikia.com/wiki/Rammus

Active: Rammus tucks into a ball and begins to roll for up to 7 seconds, losing the ability to use basic attacks or use Puncturing Taunt but amplifying his movement speed by an additional 20% each second, up to a maximum 140% increase. Upon colliding with an enemy, Rammus knocks back all nearby enemies, dealing them magic damage and slowing them for 3 seconds afterwards, also ending Powerball in the process. Powerball can be canceled instantly by reactivating the ability, or 1 second after activating Defensive Ball Curl.