The longest running RPi I have has run continuously for over 5 years. The big secret was not to use the SD card at all[1], I mount all file systems over the network to a NAS device (TruNAS from iX systems). It has a "UPS" in the form of a USB battery pack that is both charging from main power and powering the Pi. When power goes out the battery pack takes over, it has about a day of "hold up" time depending on Pi power usage. It is 'hard wired' to the local network (it doesn't use WiFi).
I got there from having SD cards (nearly[2]) always be the failure point. Everything else has been pretty reliable when used within tolerances.
[1] It does "boot" from the SD card but that acts kind of like a third stage bootloader which loads and boots the "real" OS (FreeBSD) from the NAS device.
[2] I have had one fairly spectacular looking "melt down" of a no-name USB power supply wallwart PSU which, to appears to have also put something like 12V directly across the USB power pins (my best guess at what the secondary winding of the xformer in the wall wart was putting out on the 'low' side)
I got there from having SD cards (nearly[2]) always be the failure point. Everything else has been pretty reliable when used within tolerances.
[1] It does "boot" from the SD card but that acts kind of like a third stage bootloader which loads and boots the "real" OS (FreeBSD) from the NAS device.
[2] I have had one fairly spectacular looking "melt down" of a no-name USB power supply wallwart PSU which, to appears to have also put something like 12V directly across the USB power pins (my best guess at what the secondary winding of the xformer in the wall wart was putting out on the 'low' side)