Andrew McCutchen signed his extension after his second full season in the majors.
Starling Marte signed his extension after his first full season in the majors.
It wasn't the best extension, but Jose Tabata signed his deal in the middle of his first full season in the majors.
Looking at those examples, it's easy to see that we're just now entering the time period where it makes sense for an extension for Gregory Polanco. This is all despite the fact that an extension seems hopeless, since the extension topic has surrounded Polanco since before he arrived in the majors.