please remind me...

...what's a good stoploss at NL assuming standard bankroll?

If you believe that there's no such thing as a good standard stoploss, what's YOUR stoploss per table per session?

(Posting here because it's such a basic question.)

It varies.

Usually I leave when the fish leave and there's nothing but regs.
Sometimes I leave if I lose one BI, other times 4 BIs. It depends on how I feel, if I know I'm tilted and am not playing my A game then I'm out.

It can depend on a lot of things. A basic one is the BRM guideline you've decided to use. Let's say it's 30 buyins. Any time you lose enough so you no longer have 30 buyins, it's time to leave. You can continue playing, but at a lower level.

As for other times, I don't use a fixed number. I play as long as I feel I'm +EV, which includes factoring in the opponents, my mood, etc. That said, I'd probably stop anyway if I lost 4 or 5 buyins.

I'll play until I know I'm not playing 100%

I don't do stop-loss. If I'm down 5BI but I feel like I have an edge I carry on, if I feel like I'm getting tilted which is causing me to play sub optimally I go have a game of FIFA.