Let's assume this raise will affect 250k workers this coming holiday season, bumping their pay from $12 to $15 (+$3).
Over 12 weeks at 40h/week, this represents an additional $360M in labor cost.
Using AMZN's 2017 revenue of 177.9B, this equates to 17.7 revenue-hours.
They aren't. They're comparing to revenue per hour. It doesn't matter what period of time you calculate revenue per hour with, you always get the same number.
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?
In any case, it's not clear that revenue hours is a meaningful comparison anyway because it ignores how much of their cost is labor vs anything else.
Amazon's 2017 financials showed a final net income of $3 billion. They do reinvest aggressively in their company so a fair chunk of their costs are going to be 'voluntary', but that voluntary expenditure is the very action creating all these jobs. In any case, this is going to be a very substantial cost to them.