That time is now! The Xbox Insider Program is grouped by participation. Alpha testers have already received the initial Windows 10 Creators Update builds, bringing Cortana improvements, dashboard speed increases, and an all-new Xbox Guide menu.
Microsoft announced that later today, testers in the "Beta" ring will receive their first 1703 build for testing, feedback provision, and super speedy console browsing.
Brad Rossetti from the Xbox Insider team announced that the first Beta ring build will arrive at 6 PM PST. Alpha testers will not receive a new build today. Testers in the third and fourth rings will have to wait a little longer to get their hands on the new features.
If you're familiar with the Windows 10 Insider rings, the closest approximation is: Alpha is the Fast ring, Beta is the Slow ring, and the third and fourth rings are effectively two Release Preview rings. For more information on how the new Xbox Insider Program works, head over here.
It seems likely that beta testers will receive the more stable 1703.170128-2057 build that hit the alpha ring a couple of days ago.
If you're in the Beta ring and are receiving a Creators Update build for the first time, see the link below for a list of the update's major changes (so far).