The freeze cycle normally lasts 6-8 months. (Edit: It has multiple stages, some of which might not affect you at all.)
One thing to be aware of if you choose Testing or Unstable: They’re generally not covered by the Debian Security Team, so security fixes might not arrive as consistently as with Stable.
You could always just start with Stable and plan to migrate to Testing or Unstable if you ever have a need. Stable + Backports is a good middle ground for some people.