Answer
About 2.9 ms. In review, anything from 1.93 to 4.35 counts as right.
The worked estimate
0.9 × 1 ms + 0.1 × 20 ms = 0.9 + 2.0 = 2.9 ms. The 10% of misses make up two thirds of the average, and they set the p99, so improving the miss path often matters more than speeding up hits.
Numbers: Numbers to know.