I found the article again CiA = Can in Automation

they say:

Rules of thumb for the maximum length of a unterminated cable drop Ld
and for for the cumulative drop length Ldi:

Ld < tPROPSEG / ( 50 * tP )

tPROPSEG : length of the propagation segment of the bit period
tP : specific line delay per length unit
Example: bit rate = 500 kbit/s: tPROPSEG = 12 * 125ns = 1500 ns; tP = 5 ns/m

Ld < 1500 ns / (50 * 5 ns/m) = 6 m;

Total sum < 30m

while I don't completely understand the calculation, I feel secure to accept smaller stubs

Werner