At what point does cooldown reduction cap out? Is it possible to hit the cap with only masteries and runes?
Cooldown reduction (CDR) has a hard cap at 40%.
You can get 10% CDR from masteries alone (9/0/21), and 7.50% from flat cooldown glyphs or 10% from scaling CDR glyphs, a little more if you invest in CDR seals or marks (though they will provide a lesser amount than the glyphs.
If you're playing Summoner's Rift, the Crest of the Ancient Golem provides a buff giving scaling mana regen and a flat cooldown reduction bonus of 10%.
You can hit the cap with a 9/0/21 mastery page in conjunction with scaling CDR glyphs, a blue buff, and an Elixir of Brilliance (blue pot).
However, you cannot hit the cap using only masteries and runes. You can get up to 32.5%: 10% from a 9/0/21 mastery page, 1.8% from flat CDR marks, 3.2% from flat CDR seals, 10% from scaling CDR glyphs, and 7.5% from scaling CDR quintessences.