Completely by himself I think the max was 6 hours. When we were on our own, I worked part time, and while I was at work, Kaji was in his ex-pen with food, water and bed. Now that I work full time, the most he is alone is 2 or 3 hours thanks to mom, dad, and brother.
I'm so lucky to have them. My dad is the property manager/maintanence of the apt building where we live, so he works in our building. He's in and out constantly so Kaji has someone with him. My mom is the same, constantly in and out of the apt. My little brother works from home on the days he doesn't go to school. My little man is so lucky to have them with him all day until I get home. I dread moving out, but so look forward to it!
When I go out, I make sure someone is home if I know I'll be gone for more than 2-3 hours. For the most part, if I go out I take him with me. Back when BF still lived in the states, he would be ok with Kaji tagging along on movie nights. Or we'd just hang out at his house, and of course Kaji was allowed to come over then too.