There is no standard but too long can cause just as many problems as too short. Most jacket style BCD's have a 19-inch to 22-inch corrugated inflation assembly, and therefore require a 26-inch to 28-inch low-pressure inflator hose. Similarly, the convention for most back inflation wings is a 16-inch corrugated inflation assembly and the usual low-pressure hose is 22-inches long.

Some of this is further affected by your preferred size tank and mounting height along with the design of the first stage body, fixed or swivel, plus the choice of orientation and installation port such as side or 5th port. If you are using a BC integrated second stage, selecting the appropriate length can be even more critical. Often the correct length hose will appear to be too short while your assembled rig is sitting in the tank rack, but then once you don your setup and make your water entry everything will lay correctly. Ultimately, the best answer is determined by in-water trialing to find what produces the most streamlined configuration specifically for you.