Scott, it depends on the kind of bamboo it is. With the native canes, switch cane, river cane and hill cane, the life cycle of each culm(cane) is 3 years. The first year the culm grows to its ultimate height. The second year the culm matures(walls thicken)and after the third growing season the culm dies. You want to harvest the cane after the second growing season but before it dies after the third season.
The native canes have a paperlike sheath that surrounds the node to protect the bud that grows from it. When the cane is mature that sheath begins to deteriorate and then it disappears altogether. Any time after the sheath begins to deteriorate is the time to harvest that culm.
A 3/8" open end wrench is a good guage for cutting cane for arrows. My finished hill cane arrows are 30" long and are about 3/8" or a little less at the point and 5/16" at the self nock.