How long can I leave a new puppy (lab) alone? With my job, I will be gon 8 hours a day.

8 hours is a bit long to leave a new puppy by itself. How old is the puppy you are planning on getting? Will anyone else be home or able to check in on the puppy and give it a little attention and potty breaks? Could you on your lunch breaks?

Alone just roaming around? In the house or outside? In a kennel cage?
What's your plan?

I would probably say 4 hours max. and in a kennel, you don't want him/her running around your house chewing and eating everything and you definitely don't want him having 'accidents' while your out. Maybe on your breaks go home and feed/let him outside.

If you are crating your dog the general rule of thumb is one hour for each month after the dog is 2 months old...

So if your dog is 3 months old, it can stay in a crate for 2 hours,
4 months old, it can stay in for 3 hours... etc...

Just remember that a young puppy can get bored very easily and can become destructive.
They only have small bladders and if you are leaving it alone for 8 hours expect to come home to pee and poop.

Toilet training will be more difficult if as soon as you get to pup you will be leaving it alone during the day so make sure you do strict training before and after work.

I leave my dogs in a room when I go out, I prefer this to crating as it gives them some sort of freedom in case I am home late.
Just make sure it is completely puppy proof, somewhere like a kitchen or bathroom with no cords hanging down would be good.

Always remember to leave fresh water out and plenty of toys to alleviate boredom