Oh..."transit period" refers to the time surrounding the exact transit. As a rough measure we could say that when the transiting planet is within 3 degrees either before or after the exact transit it is "effective" and can "cause" things to happen. So the slower the planet, the longer the transit period. When a transiting planet turns retrograde just after passing over a natal planet, the period is greatly extended (it lasts a much longer-than-normal time) and is quite powerful because, in most cases, there will be three contacts (exact transits) instead of just one.