When should I stop farming as nasus

when you 1 shot the enemy nexus

In all seriousness, if your team is being crushed 4v5, stop it and try to tank damage in TFs and make use of your E.

If there is some scumbag playing Tryndamere, never leave him alone and farm as much as he does.

If you have TP and can predict a battle soon, follow your team and then go back to top lane to push (assuming you win the battle)

To me, having a 300+ stack Nasus on my team with a TF is already good enough, but some people are greedy and need 800. Please don't be that guy who spends the whole day farming top lane and then flames his team for losing. Balance your doge today!

If #stacks = Infinity, then and only then may you stop farming.

You never stop.

The answer is more of "when" and "where" does your teammate need you.

I think the best answer is, you should never stop.

But you need to know when to take a break and use those stacks to clobber someone.

If youre farming well you can hit 200 stacks by 10 minutes. After that just play normally and help your team. Stop farming when your team needs you, thats the simple answer.

300 is the minimum stacks needed. After that, you'll still need to continuing farming your Q to gain more advantageous damage output. And also, with good Q farming, there's not really a need to build AD damage Nasus. Tanky build would be good enough.