Estimated S/N given the SPI data set (Sect. 3) and the range of plausible ²²Na ejecta masses for our sample of 97 novae. Top: the S/N of individual objects does not increase with because the exponential decay is faster than the accumulation of 1275 keV photons (cyan lines). The cumulative signal of point sources saturates on a timescale of a few years (dark blue). If all objects are assumed to be ONe novae, the S/N would be about three times higher (pale red). Bottom: comparison between cumulative point-source emission significance and diffuse-emission significance during the INTEGRAL mission timescale. The bands contain the 68th percentile from 1000 realisations of the population synthesis model (boundaries marked by dashed lines, median by solid lines; Sect. 2.4), to distribute ejecta masses among sources and diffuse emission.