A ticket is valid for 30 days after they are first generated. After that time if the ticket has not been used you will have to create a new one.
Most tickets are for forms that are intended to be filled in fairly immediately - it is thought that feedback or assessment from an observation that happened over a month ago is not very educationally useful.
A fair proportion of tickets remain unused and expiration provides a way of preventing the system from being cluttered up with old tickets. Expiration also discourages over-enthusiastic trainees from indefinite reminding of would-be assessors.