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 5.89% from flat cooldown 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 scaling mana regen and cooldown reduction bonus, which caps out at 25% at level 18.
Therefore, using flat CDR glyphs, and a 9/0/21 mastery spec, you can cap cooldown reduction (and .89% to spare! without any items at all.