What are the design considerations for showing timers in forms?

By timers, I mean a ticking countdown clock for time sensitive tasks, such as holding a reservation spot. Essentially "You have 3:00 minutes to complete your registration".

I've seen Eventbrite implement this and was wondering if there were any best practices or considerations in terms of usability.

This is what I have:

1. The user selects a time slot, countdown appears near the CTA
2. Changing time slots resets the countdown
3. Countdown appears near the CTA in all other steps of the step-form
4. If countdown expires before user submits the scheduling, notification appears telling them countdown expired with a CTA linking back to time slot selection step.