Collaborators have 72 hours to open the itinerary you have sent them. If they click on the link after the 72 hour time frame the link will be expired and they will see this error message:

Sending them another invite to collaborate will send them a new link they can use to access the the itinerary.

If they have clicked the link within the 72 hour time frame the itinerary will be in their Travefy account. They can simply open Travefy to edit the itinerary!