Answer
About 3,900 reads/s. In review, anything from 1,950 to 7,800 counts as right.
The worked estimate
A 30-day month is 30 × 86,400 ≈ 2.6 million seconds. Writes: 100,000,000 ÷ 2,592,000 ≈ 39 a second. Reads are 100× that: 39 × 100 ≈ 3,900 reads/s on average, so the system is clearly read-heavy and a cache in front of the database pays off.
Numbers: Numbers to know.