How much time is required between top efforts for a lower figure to be considered a new top?

I've been going through the archived Breeders' Cup races in an attempt to build a very basic par chart for winning speed figures in BC races, previous tops as well as whether or not the race was won with a top, pair, off or X.

An example of what I'm looking at is Bar of Gold in the 2017 Filly & Mare Sprint. She won that race with a TG 1, which the Thoro-Pattern lists as a top, yet in October of the previous year she ran a 4.25-. Should a handicapper take only the most recent 12 months when considering a new top?