Q&A

 Master

To pass the state emission tests in many states a vehicle can not have a Check Engine light illuminated with the engine running and test monitors must be passed. This prevents a person that has a Check Engine light on from having the Check Engine Light turned off and then driving straight to the test center to have their emissions test done. Some emission test are done every time your car is started, some tests need to pass certain criteria before the computer in your car analyzes the system to determine if those systems are OK. Systems like catalytic converter efficiency, evaporitive systems and EGR are only tested after a particular "drive cycle" is completed that differs slightly for each car manufacturer. For evaporative systems the gas tank may need to be between 20 to 80% full and ambient temperature below a certain temperature and above another. The drive cycles require the car be driven at a certain speed for so many seconds and then another speed or load rating on the engine for a certain time. To ensure the monitors are completed as quickly as possible, a repair facility with a dynometer/rolling road run the car according to the vehicle manufacturers criteria to get all drive cycles enabled.
The answer to your question is, for a customer to drive their car and pass the drive cycle, monitor readiness may take a week to a month depending on the driver and driving conditions. Plugging in even a generic scan tool tells you which monitors have been passed and which have not. Evaporation emissions system tests are the one that are hardest to meet the criteria to pass the monitor.
After a emission related repair more expensive scan tools have bi-directional control to cycle solineods to test components in the various systems.
I just worked on a customer's Ferrari 355 yesterday it took him 2 years to have a "code" reset for low catalytic converter efficiency (a car which he frequently drives) but that is an exception.