As per the cisco Standards yes it is 50ms
here is the reference for IOS and IOS-XE.
your question is appropriate.
For example in IE2000 datasheet the G.8032 standard is not mentioned but only support of Cisco REP is declared.
There is table 8 a section about conforming standards but only IEEE standards and RFCs are reported. There is not a section about ITU-T standards. I think it should be added at least for industrial ethernet products.
Be aware that REP is a specialized protocol working only on a single ring topology.
The second document link provided by BB explains the protocol messages and how it achieves so fast convergence.
Hope to help