Say I have a 5k bankroll for 1/2 NL, I sit down for a session and walk away up 150$. How much should I keep for myself and how much should I add to the bankroll? Is it even necessary to continue adding to it when 5k is a sufficient amount of money to handle the swings for the level I'm playing at?
As with all answers in poker it depends.
Currently at $5,000 you have 25 max buy-ins of $200 each. I've read that 20 max buy-ins being $4,000 is the minimum. Personally I would try to reach the 30 max buy-in level so you would have $6,000 so I would add 100% of your profits until you reach the $6k level.
After you get to the 30 max BI level then what are you plans? If you NEVER plan on moving up in stakes then take 100% of the profits that you have over your $6k BR. If you dip below $6k then 100% of your profits should go to getting to that $6k level again.
If you have plans of moving up to the next level then I would take 50% of your profits & use the other 50% to start adding to get to the next level.