They only regenerate when needed.
There are sensors in the exhaust to detect when it needs doing.
EOYLS are injected in to the combustion chamber with diesel to raise the exhaust temp cos the smaller engines have difficulty getting to the required temperature to regen the DPF as what i was told by the master tech at volvo.
on later cars the engine mapping was changed for the removal of EOYLS.
as alex said the car will regen as and when it requires depending on exhaust pressure readings the ECU gets.