An example: let's say you've got four loops: 0:30, 0:27, 0:11, and 0:18. Start them at the same time, and then the next time those will line up in that way will be nearly 45 hours later (30 x 27 x 11 x 18 / 60 / 60 = one period of all loops). Pretty effective, really.
-- Lugia
The math is a bit off here: the least common multiple of 30, 27, 11, 18 is 5940, so the loops will repeat after 99 minutes (which is still pretty effective). Changing the lengths to 31, 29, 11, 19 will get you 52+ hours.
The least common multiple of two numbers is their product divided by their greatest common divisor. The greatest common divisor can be computed by Euclid's algorithm, probably the oldest recorded algorithm (circa 300 BC). Euclid's algorithm can be adapted to evenly distribute beats in a bar, leading to the Euclidean rhythms one hears about in the modular world.