Its such an odd question. Almost as bad as the one about monkeys and Shakespeare's Hamlet. But its not that hard. Its a bit mathematical, though.
First we must start with the basic premise that time flies when you (the metaphorical you, not you personally) are having fun. Then we extrapolate it to say that the more fun you have, the faster time flies.
This obviously means that:
Actual Time - Perceived Time = Fun (measured in seconds lost)
The next step in this experiment (it's the best part) is to play with one monkey as a friend uses a stopwatch to time you. When you are finished, you tell him your Perceived Time and he records the Actual time. Then you use the above equation to find out how much fun your monkey is.
Now comes the sad part. Put the monkey in a measured amount of water. You'll have to drown the bugger because he'll thrash too hard for you to tell how much water he displaces. I know, I know, but they're MONKEYS, the FDA hasn't approved this procedure for human trials. Once this is done you'll know two things: How much fun is 1 monkey, and what volume is 1 monkey.
Now, when your barrel of monkeys arrives, you must find its volume. best to use the formula for cylindric volume; barrels float, and if you think one monkey thrashing is hard to drown...
Now some math:
Volume of Barrel / Volume of 1 Monkey = No. of Monkeys in the Barrel
How Much Fun Is 1 Monkey x No. of Monkeys in Barrel = How Much Fun is a Barrel of Monkeys