It's triggered by sensors. How often it actively needs to regenerate depends on use and where in the exhaust system the DPF is located. If it's bolted straight onto an integrated exhaust manifold on the engine, then it will passively regenerate more and rarely need to actively regenerate. If it's half way down the exhaust pipe it will need to actively regenerate using post injected fuel fairly often and the engine must never be switched off while this is happening. Unfortunately no manufacturer fits a light to tell you, but if you open the car door and smell heat under the car, then it's actively regenerating.