Thanks for reaching out to the Zoom Developer Forum, I am happy to help here!
The “start_url” will expire 2 hours after the creation of the meeting. If you want to get an updated value for the expired “start_url” you can retrieve it by making a call to the Get a Meeting API .
You can not modify it to make it longer but you can retrieve a new one by calling Get a Meeting API.
I hope that helps!
Best,
Elisa