Anything that stimulates blood flow works. Whether it's for 20 seconds or 10 minutes that's blood that wasn't flowing before. This is why growth aids like peppermint oil & cayenne pepper work because they stimulate blood flow.
You can even hang upside down. That will work too. The time you do it for is based on the results you want and what you are willing to give. Plus, hair has a maximum growth rate of about an inch a month. You won't get much more than that no matter what you do.