It’s a difficult one to answer exactly, but in my experience when SNR drops below ~15 (as measured in the b=0 images within the region of interest), things start to look a bit messy. My suspicion is that at this SNR level, and when looking at b ≈ 3000 s/mm², the SNR in the DW images is getting too close to 1 for comfort. At this level, even lots of measurements won’t necessarily help all that much since the Rician bias will wash out the contrast.
As to where to find that information, well to be honest, there isn’t really any particular reference for that. Part of the problem is that there is an interaction with the Rician bias, but also with the b-value, the number of volumes acquired, the use of parallel imaging, etc, all of which complicates things a bit. So my SNR > 15 recommendation really is just my personal experience…