Pretty close to 100%.  Other than occasional algorithm configuration changes, and the rare power outage, mining is a lights-out operation for me.  I took a two week vacation, with no access to a computer or to my rigs, and everything ran smoothly.  I can remotely monitor via my phone, but it's really not needed.  I get notifications if a rig is down, but by the time I've checked the status of the system in question, it's either already running again, or is in the midst of rebooting.  I've got monitoring software running on a Raspberry Pi connected to the rig network, and if it detects that a rig has not been responding after a certain amount of time, it sends the command to the PDU to power cycle that outlet.

With prices at their current highs, even a half hour of downtime is costly, so I really like having as much monitoring and recovery as possible completely automated.