An aftertreatment system for an engine on a work machine may include a selective catalytic reduction (SCR) system configured for arrangement downstream of the engine and having a dosing system for dosing the SCR system with treatment fluid. The aftertreatment system may also include a NOxin sensor arranged at or near the inlet of the SCR system, a NOxout sensor arranged at or near the outlet of the SCR system, and an SCR catalyst monitoring module configured to periodically establish a maximum NOx conversion capacity of an SCR catalyst in the SCR system. A method for determining a maximum conversion capacity at a selected number or working hours and/or determining a remaining useful life of the SCR catalyst is also provided.