We present estimates of CN and Mg overabundances with respect to Fe for early-type galaxies in eight clusters over a range of richness and morphology. Spectra were taken from the Sloan Digital Sky Survey Data Release 1 and from William Hershel Telescope and Centro Astronómico Hispano-Alemán observations. Abundances were derived from absorption lines and single-burst population models, by comparing galaxy spectra with appropriately broadened synthetic model spectra. We detect correlations between the [Mg/CN] and [CN/Fe] values and the cluster X-ray luminosity. No correlation is observed for [Mg/Fe]. We also see a clear trend in the richness and morphology of the clusters. This is interpreted as giving varying formation timescales for CN, Mg, and Fe and a varying star formation history in early-type galaxies as a function of their environment: intermediate-mass early-type galaxies in more massive clusters are assembled on shorter timescales than in less massive clusters, with an upper limit of ~1 Gyr.