Should have started saving a while ago :P I read ahead before I started, saved all my IP from level 1, used RP for three champs and had some spare(enough for 6days worth of xp boost). By the time I dinged level 21, I had about 21k IP, which was enough for 2 rune pages and some spare.
Start saving now. A set of quints is around 6k, then everything else varies, but is also expensive. Runes cost a lot. So start saving now.