Answer
A 30-day month has about 2.6 million seconds, so 100 million requests a month average about 40 requests a second.
Also accepted: 2.5 million, 2,600,000, 2.6M for 2.6 million; 38, 39 for 40.
Why
30 × 86,400 = 2,592,000 s. 100,000,000 ÷ 2,592,000 ≈ 38.6, so "about 40" is the quick answer. Monthly figures are common in requirements (new URLs, uploads), so this conversion comes up often.