I am wondering if anyone knows what the code says about the distance from a Distribution panel to a MLO only panel, at what point am I required to use some sort of protection device?
Assuming the distribution panel has the properly sized protective device and voltage drop is dealt with, there is no NEC requirement to have a main circuit breaker in the downstream panel, regardless of distance. A MCB would be for convenience only.
There is no such code requirment as far as I know. It should be a matter of professional jusdgement and practicality.
Question you need to ask yourself is, if you were to work on this panel, can you rely on the remote breaker being positively off for the duration of the work? If you dont want to be that person, provide a local disconnect or MCB.