Hi,
- Impossible to determine hotfix upgrade time. It depends the each HF. Some HF must to do reboot, Some HF isn't required to do reboot.
- If I were you, I'll never use the Auto-Update. I prefer update manually method and update Secondary gateway first then after update Primary member. I think it is Best Practice
- HF is the fixing errors and improving the system. But should thoughtfully while do install HF. Do only recommended HF. /Once up time, our customer installed Clean Install Image from CPUSE/
- No affection on your network. Maybe 1 or 2 second spend to handle your standby appliance.