My current effort is also a side business while I continue to work my FT job. It is definitely detrimental to the business to continue to devote 40+ hours per week to something else, but I need to feed my family (if I was young and single with no other financial obligations, I would totally just quit the day job and live on canned meat and potatoes) :/
My advice - if you're young, take your financial risks now. It only gets harder to do later :)