Review chapter 1 of publication 463, but I don’t think you have to work a certain number of hours to count as a work day instead of a personal day. At least, there is no minimum requirement in the regulations. However, I would think that if you just posting a few pictures per day on instagram, there would be very little difference between you doing it as a business and someone else posting their vacation, and the IRS might scrutinize it if you have the bad luck to be audited. Every situation is judged on its own facts, and the question would be whether your expenses are the ordinary and necessary expenses of your profit-making business.
The best way to get an answer that is specific to your exact needs is to contact your own professional tax advisor.
