You can claim Founder pricing at any time while slots remain — Founder 100 doesn't expire on a date, it expires when the 100th slot is taken. As long as the live counter on the pricing page shows spots left, the "Claim Founder pricing" button is live.
What happens if I cancel?
The founder rate is anchored to your active subscription, not to your account. If you cancel your subscription, the founder rate does NOT auto-restore when you come back. You'd re-enter the standard checkout flow — if Founder 100 still has open slots you can claim one again (and consume it); otherwise you'd pay list price.
This rewards continuous subscribers and prevents churn-and-rejoin gaming. Most people will never notice — keep your subscription active and your founder rate is locked in for life.