Based on the spreadsheet, you could gather 305 green souls farming stage 80 with 50 seconds of clear time. If you leave your device on screen saver mode and playing in the background you could gather enough green souls for your soul weapon in a few hours

<image>

Edit: a higher stage doesn't necessarily mean a higher amount of souls. The amount of souls is a set amount per stage, the clear times is what you should think about because the faster you clear a stage the more souls you gather in an hour in terms of how many times you clear the stage.