You have 6 months after your Training Plan end date to complete any sessions that you miss.
After the scheduled end date, planned workouts will no longer appear in the Activities tab of the mobile app.
6 months after that end date your Training Plan will no longer appear as current on the website and will end up in your Past Training Plans.
If for some reason, you're not quite ready to finish your Training Plan by your end date, you can push your plan forward to give you some more time. You can learn how to do this here.