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. I'd consider dropping 2 or 3 bars in each form as cheap insurance.
For example, consider the concrete light post bases you sometimes see in department store parking lots. Those are often poured using form tubes. They protrude several feet, or roughly the same height as their diameter.
In your case I'd probably limit protrusion to 1.5 times the diameter, or about 15". If you need to go higher, use a 12" tube or rebar with a smaller tube.