30' or less in warm clear water no overhead environment in a place I am comfortable and experienced with I don't bring an extra tank (primary may be a 63). In the same environment going to 80' I'll bring a 6 and 80 as a primary. Greater than 80' to 120' I bring a buddy and maybe the 6 as well. On almost all my deeper dives I only stay at the deepest depth for a couple minutes.