Ultimately it will vary as to how long Polytrol will last depending on UV levels, weathering as well as pollution that the treated surface is exposed to.
Generally speaking, Polytol should last at least 12 months before needing another application. However, it should be noted that treated surfaces exposed to high levels of UV and weathering (by the coasts for example) will likely require another application earlier.
On the opposite side of this, surfaces that are not exposed to these types of conditions likely won't require another application of Polytrol for 24+ months!