Every 1 released (6300), the last 6300 goes to 4800. (That means that even though you lose 1500, you still gain 6300 so at average, you gain 4800 IP needed per every released champ.)
Every 3 champions released, a 6300 and 1 other get reduced. (So with the total IP amount needed for all champs being raised 4800, a 4800, 3150, or 1350 will be reduced. The most reduction is the 3150 to 1350 (or 1800 less needed IP) so the total champion IP amount is only raised 3000 IP points or at most 3900 (if it is a 1350 to 450 which is only a 900 IP difference).
Every release brings more IP. Whether 4800 or 3000 the amount steadily grows. They are nearing 800,000 needed IP for all champions. This does not include runes. When you can only get at average 100 IP per win and 70 IP per loss, it will still take new players 8,000 games (won games I might add) at most to just gain all of the champions.
I've been playing for 2 years, and still have 44 more champions to get.