It is not so much absolute figure, but I usually use something between 0.1 - 0.25 dB in linearity sweep. Usually you can see where linearity begins to go off from a straight line, then round it down to nearest bit.
For example for Holo Audio May (and Spring) you can see it starts to go off at -120 dB:
Corresponding to 20-bit.
You can then further confirm it with measurements using for example -120 dBFS tone with full resolution and then the determined resolution combined with noise shaping to verify that things work as expected. Here example from Holo Spring 2.
24-bit TPDF dithered input:
20-bit NS9 noise shaped input:
