8 weeks is the ideal time to get the most out of your release plan. You can still set up a release cycle if your release date is sooner, however, it’ll be a bit more difficult to get everything in place for release day and you'll miss out on the earlier actions.
For example, if your release was in 5 weeks you would miss out on the first 3 weeks of suggested tasks.
What we would recommend in this situation is to create an 8 week release cycle, and once you’ve completed the early actions you can then delete this one and set up a new release cycle with your planned date. This way you can get all the benefits of the feature!