Just trying to put some reasonable bounds on this estimate. I assumed 12 weeks is "the holiday shopping season" where Amazon has a lot of extra workers who would benefit. And I guessed 250k of them getting $3 raises. You could also guess at the total cost in other ways, many of which would be superior.

The overall goal of these lazy estimates is to understand how significant a cost this is for a company like Amazon. Could activists have hoped for more? Are other companies, for whom the costs would be felt quite differently, likely to offer something similar?

You have to use some amount of time to get a number measured in dollars. What amount of time do you want to use?

Think of it like this, 12 weeks of extra labour cost is offset by 17.7 revenue hours, you could also say 1 weeks extra labour cost is the equivalent of just under 1.5 revenue hours.

12 weeks is (approximately) a business quarter. Corporate finances are most commonly aggregated across a quarterly schedule.