What you're really asking is how far your footing can protrude from the ground. Once the concrete is set, the form tube is irrelevant. In fact, you'll probably want to cut it off at ground level so your footing doesn't look like a decomposing cardboard box.
Concrete can protrude any reasonable distance from the ground, as long as it reaches below frost depth (from grade, not from the top of the footing). That said, at some height your concrete footing becomes more of a post, and you'd probably want steel reinforcement (rebar) in case of cracking.