The FAQ describes two types of tests. One is a test where you basically use the solar system as a gyroscope. In this type, I think the answer to your question is that we do want to use observations of the solar system over the longest possible time, in order to see a cumulative effect that is as big as possible, but the longest possible time is only 100 years or so. In both cases (GPB and solar system), what's working in your favor is that the effect is cumulative, so you want the longest possible period of observation.
The other type of test described in the FAQ is CMB tests, which are completely different.