MTTR = (Total downtime hours) ÷ (Number of failures). It is bounded on the low end by how fast your technician can arrive and on the high end by parts availability.
Restaurants with strong spare-parts strategy and vendor SLAs see MTTR figures as low as 1-2 hours on common failures (door seals, thermostats, fan motors). Without those, the same failures stretch to 24-48 hours waiting on parts.