How fast will the sun grow when it becomes a red giant? We all know that our sun will become a red giant in a couple of billion years? But how fast will the process be when it starts? Would it be theoretically possible (disregarding the deadly effects of this event) for a human to experience the sun as its present size until it swallows earth?
Yes, the time is very long, but exactly how long depends on when you "start the clock" for the expansion to a red giant.  If you take the end of the "main sequence" (when the core hydrogen runs out) as the start of the expansion phase, it's more like a billion years to complete the process of becoming a maximally expanded red giant.  But the answer that gets 5 million years focuses only on the phase of most rapid expansion (which begins after the star is already quite a bit larger than the Sun).  Either way, our lives a the smallest blink of an eye to a star!