Hiking conditions are usually the best in late June-early September in the Indian Peaks Wilderness. Please buy your permit if you are backpacking during the summer. In the winter, no permits are required. Campfires are prohibited at Caribou Lake.
The earlier or later in the year, you will find that there are fewer people in Grand County during those times. Narrow switchbacks between Arapaho Pass and Caribou Lake may be difficult in snow/ice so travel carefully in these conditions. The weather tends to change very quickly up here so be prepared for anything.