It is 2.75 lb., center cut. I have seen some recipes that call for just 3-4 hours. But what it be bad if I did it for like 8-9 hours? It's just that I am not going to be home at the 3-4 hours point so if I do brine it, it'd have to be in the brine for 8-9 hours. Would that make the pork too salty / bad?
If you have never brined before then I would warn against going that long. Someone who is experienced with brining and would have a good handle on how much less salt to use could get away with longer brine times, but for someone not as experienced it could ruin your meat. Over brined meat gets a nasty texture and many people have brined once and overdone it and never brined again because of that experience.
Personally I would not go more than 4 hours. I think for a loin 3 hours would probably be ideal.
The ONLY time I feel the need to brine is when I'm putting something that cooks fast (like chicken breast or pork loin maybe) on when something tougher takes a long time at a much lower temperature than I would normally cook the chicken or loin.
Otherwise, you don't need to brine. Cook your pork loin to 140 (145 if you must) let it rest for 10 minutes, and it will be perfectly done - moist and juicy.