No, your 2ndLine number doesn't expire as long as you maintain an active subscription.
Here's how to check your subscription status:
On iOS device:
Open the Settings app.
Tap on your Apple ID.
Tap on Subscriptions.
Find "2nd Phone Number App" and tap on it.
You'll see your subscription status and renewal date.
Remember:
If your subscription expires and you don't renew within 7 days, your number will be released and available for others to claim.
Renewing your subscription on time ensures uninterrupted service and protects your number.