Maximum? I don't think there is a maximum time that you can leave your system up.
I've had my system on for about 6 months, I guess. I would shut it off if I have to leave home for more than a day or two, otherwise it is simply on, and asleep if I am not at the computer. That's not the same as 'uptime' as I have restarted various times during that period, but my Mac is almost always on, usually sleeping.
The main thing to consider is the reliability of your power. If you don't have a UPS, and your local power is not too reliable, then I might recommend shut down, and not sleep.