Excimer laser radiation changes the physical and optical properties of fused silica. These changes include compaction of the glass and induced absorption, both of which have an impact on the expected lifetime of silica lenses used in optical microlithography. We report on our ongoing study of excimer laser induced changes in fused silica. We use a fully automated experimental setup designed for marathon exposure of the sample at low fluence. In each setup, using either an ArF or a KrF laser, up to five samples are exposed simultaneously and their induced absorption is measured in situ. The spatial and temporal profiles of the laser beam can also be measured in the same setup. We present and discuss results from marathon test of fused silica at fluences close to the conditions expected in optical microlithography systems.