Do unused credits carry over?

Plan credits: no. Extra credits: 90 days.

Plan credits do not carry over. At each monthly refill your plan balance is set back to your plan’s monthly amount: if you had 300 left on a 1,000-credit plan, you start the new month with 1,000, not 1,300.

Extra credits from packs are separate. Each purchase is valid for 90 days from the day it was added, and what is left after that expires. Billing shows which credits expire next, and when.