So I don't know of any specifics related to Coop (as compared to PvP), but I do know that Riot recently changed their formula. If I understand the new formula correctly, there should be no difference between PvP and PvE IP/XP gains on a per game basis (obvious caps still apply).
Previously, the system was based around providing IP in comparison to the 20minute mark (also known as the surrender minimum time). As you played towards that mark you gained more IP/XP and after that mark, these gains rapidly diminished.
To help explain this, let's take the following sample:
I play a PvP against Grace Note and she beats me 25 minutes in. She gets about 115 IP (plus a bonus) for winning and I get about 65 for losing. Now, let's say we play again and again she wins, but this time it takes her 45 minutes to woop my butt. I would get 72 IP for hanging in longer, but she would only get 100IP!
Obviously the goal here was to penalize longer wins and shorter losses. Unfortunately, this encourages farming games, where teams agree to surrender at the 20 minute mark. So Riot changed that formula to a new one. Here's your TL;DR:
IP/XP scales with time
For those concerned about endless games, the formula is essentially linear up till the 55 minute mark. Additionally, both Winners and Losers scale up with time (meaning you're no longer penalized for a long win). There is also an extra flat bonus for playing the game which does not scale. Finally, they've also removed the bonus for non-leaver streaks (in preference to the new judgement system.
For more information on the new system you can read here.
For those unaware there are also difficulty scaling modifiers for Coop vs AI
These percentages represent how much of the standard reward is received:
- Summoner level 1-9: Beginner: 100% Intermediate: 100%
- Summoner level 11-19: Beginner: 75% Intermediate: 100%
- Summoner level 20-29: Beginner: 50% Intermediate: 100%
- Summoner level 30: Beginner: 25% Intermediate: 75%