Per-win IP boosts (the kind most often given out for free) do not change in value based on the length of the game. Do not intentionally draw out winning games (and thereby risk losing those games) in the mistaken belief you will net additional IP.
Per-time IP boosts double the base IP gained for all games played, which is a function of game length, but per-win boosts reward a constant value on each victory, which varies with game type.
A per-win boost will always award:
- 145 IP for 5v5 (PvP) game on Summoner's Rift,
- 96 IP for Howling Abyss,
- 74 IP for a Co-op vs. AI game.
I don't know the Crystal Scar or Twisted Treeline values, but I've been led to believe that they are each less than PvP Summoner's Rift, but more than vs. AI Summoner's rift.
In short, the way to maximize a per-win IP boost is to exhaust the boost on Summoner's Rift before you play Dominion, Treeline, or especially Co-op vs. AI.