It seems likely that working more will increase your expected future income. That's obvious if we're talking about overtime, or a paid side project. What I have in mind here is subtler: putting more time into work and work-related activities makes you better at your job, so you get promoted sooner, or generate better-paid options outside your current organisation.
So imagine you spend an extra 10% of your time on work or work-related activities — the job itself, self-development, networking, and so on. How much do you want to assume that raises your income? That's the first dial, and everything else follows from it.
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Where is the discount rate? There isn't one, deliberately. Both sides are ongoing flows — you put in the extra hours every year, and you earn the higher income every year — so the same discount factor sits on the top and the bottom of the ratio and cancels out. If a perpetuity is hard to picture, run it over ten years instead: ten years of extra hours against ten years of higher income gives you exactly the same number per hour.
The other way to use this. The obvious application is life admin. But the same number is your willingness to pay to buy an hour back — and the hour you buy doesn't have to go into work. It can go into simply having less to do: a lower chance of burning out, or being in shape to take a more demanding job later. That is a real return too, it is just harder to put on a dial.
Made-up numbers to flesh out a point, not financial advice.