The price you pay when you join is an introductory rate that runs for the length of your term. This page explains what changes afterward, so the first full price payment isn't a surprise.
How long the introductory rate lasts
It lasts exactly as long as the term you bought. A 3 month term has 3 months at the introductory rate. A 6 month term has 6 months.
What you pay afterward
Your plan moves to its standard rate. You can see the standard rate for every plan on the Flex Mobile here.
Everything else about your plan stays the same. Your data, hotspot, reward points and phone number don't change.
What to do before your term ends
Decide whether the standard rate still suits you. You can stay on the same plan, move to a different one, or leave. There's no early termination fee either way.
If you want to leave instead
You can transfer your number to another carrier. See the guide on transferring your number away from Flex Mobile.