I guess they discourage you for playing too many games in one sitting

It's based on game length and whether you won or lost.

So there are several things:

1. You get less IP the higher your level gets ( slowly ramping up from level 1 to 30)
2. Every map has min/max caps on IP earned
3. Summoner's Rift: 18 IP + 2.312 per minute IP for a win ; 16 IP + 1.405 per minute IP for a loss
4. There is a first win of the day system (+150 IP for the first win)

Hope that clears things up a little bit.

im pretty sure that may be the first win of the day bonus coming into it? IP gaps that big tend to be due to that or an IP boost thats been bought/given out free

+145 IP is a pretty good amount of IP to get from a game with no first win of the day bonus.

Secondly, the more games you play in a day the less IP you get.

Thanks for the reply guys, It helps clear things up.

@manekineko: Why do you get less IP for being a higher level? That doesn't seem very fair.

It kinda is the other way around, you get more ip at lower levels to get you going in the game.

Also, I think it's worth mentioning that they do not want to reward players based on personal score. They don't want to promote scorewhoring and killstealing, only the overall outcom of the game is supposed to matter. Which in this case is based on if you win or lose. Personally, I would enjoy if teams who won quickly got rewarded for it. Since there are a lot of cool blitzkrieg/rush strategies you can pull off. Dragging games out to gain more IP isn't a cool mechanic. :/