it is the length of time money should be kept in an account. Some embassies require the 500kTHB to be in your account for 3-6 months. Many embassies have no time limit.
On the main DTV FB page someone posted about getting a DTV via drop shipping and was advised it would be tough to get.Check that group, you can see the range of jobs and businesses used to get the DTV.
Rental owners have been approved for the DTV. You show how you manage the property online, collection of rent, dealing with tenants, suppliers, repairs etc. No income requirement so your online food business is ok to use.
You need to contact the embassy you are applying from if they accept an investment account, some US embassies accept them. The majority of embassies require the 500k in a personal bank account. The seasoning requirement is also embassy specific.