There is no one-size-fits-all bus utilization limit. You should analyze your bus from the standpoint of latency constraints per every type of message exchanged over it; this question is briefly explored in the section 4.1.4 “Transfer priority” of the Specification: https://new.uavcan.org/specification/UAVCAN_Specification_v1.0.pdf; also, this thread may also be of relevance: Transfer priority level mnemonics