How to descide the running time under using continous source?

[beizhixing]

Hi, I want to simulate waveguide by using continous source, it is tricky to set the running time.

The image below are under different simulation time setting:

sum.run(until=400)

sum.run(until=500)

sum.run(until=3000)

I think as long as the simulation convergence, no matter how long I set, the result shoud be not change.

I'd be appreciate if you can help me to solve it, @smartalecH @oskooi !

[stevengj]

What is the scale bar on the third plot? Is the simulation diverging?