It takes about 410,000 IP to buy every champion. I don't know which 30 champions you have, but for someone starting fresh, it would take:
410,000 + 6300 every three weeks a new champion comes out.
Assuming you're above average and have a 60% win rate, and the average game goes on for 30 minutes (more games end at 20 minutes than those that extend beyond 50 minutes so I brought the average down from 40 to 30), you'll get 88 IP per win and 58 IP per loss (plus 150 for first win of the day). So, if you play 10 games a day (5 hours), you'll make 910 IP per day.
That's 19110 every three weeks. Subtract 6300 (assuming you buy the new champion every three weeks and use spare IP for old champions) and you have a surplus of 12810 every three weeks.
It would take 96 weeks (or 32 sets of "three week periods") at this rate to unlock every champion without spending any RP. So, if you want to have every champion, and don't want to spend 3360 hours to get them, you're going to have to spend money. One thing to note is that the champion bundles are extremely cost efficient (I think you get the first 40 champions for 30$ or so, and there's more than 3 6300 champions included so its better than spending that money on new champions).
I've been playing since the summer of 2010, and spent around $200-250 since then (getting a $35 RP card every three or four months), and unlocked my last champion early this year.
Edit: Reason I gave a 60% win rate is because I'm assuming you'll be playing normals mostly, and most of the people I know hover around 55%-60% win rates in normals due to the fact that normal Elo rises more slowly from what I've heard.
The general rule about people seems to be: "Attractive, Single, Mentally Stable," choose two.