My roommates and I usually take about a week off every two months or so, to lower our tolerances.

We found that at least for us, that two months of constant 5-6 times a day smoking a couple bowls would put us at the apex of our tolerances, and that it would start taking more weed to get the same effect from there on.

But I personally can feel the difference in tolerance after only a couple days of not smoking, so ymmv.