Max length for a digital coxial cable?

This depends on the signal and the type of cable. Standard definition digital video signals can be run for 1000 feet or more with low loss broadcast standard cable. HD is limited to around 400 feet with the same cable. However, there are always lower loss cables available to extend the maximum distance.

In the home, co-ax cables are often good for in excess of 100 feet although it depends very much on the signal source, the receiver and the cable itself.