At a certain grad school I am considering, there is a certain professor who I would be most interested in researching under, in fact if I couldn't have him as my advisor I my interest in this school would become diminished, but I read in his bio that he is a currently a visiting professor at a different university. In this case, should I still express my interest in his research in my statement of purpose? How long do visiting professorships typically last? Could I hope that I could have him as an advisor? I'm unsure how to approach this situation.
There is no point in guessing. Contact the professor directly. Express your interest in working with him, and ask if he is planning to be back at the "home" university next year (or whenever you'd be starting), and if he would be looking to take on graduate students.
A likely scenario is that he is visiting the other university while on sabbatical, which would typically last one year or less. However, there are many other possible scenarios and no way to know the true situation without asking.