Sample of 122 long GRB afterglows observed with GROND within the first 4 hr, coloured according to redshift. For bright afterglows, these are the first 4-min exposures; for fainter afterglows, stacking of multiple exposures has been applied (as indicated by horizontal ‘error’ bars) and a compromise between amount of stacking and time after the GRB has been adopted. The sensitivity limit of GROND@2.2m is given as dashed line, assuming a start of the exposure at 150 s and an exposure of 60 s, thereafter improving as time progresses: at early times, this is close to the typical t⁻¹ afterglow fading rate. For those afterglows with J-band upper limits but z′-detections, we plotted an open square at the expected J-band value (using a mean colour of z − J = 0.15 mag), connecting it with the downwards arrow with a dotted line (GRB 120328A at 6534 s has z′ = 23.79±0.35, outside of the plot). The sky background limits our sensitivity to J(AB) ≲ 22 mag, irrespective of exposure duration.