A small molecule needs 0.03 s to diffuse across 5 µm…

Step 1. 15 µm. 3 times the distance: $0.03\ \text{s} \times 9 = 0.27\ \text{s}$.

Step 2. 50 µm. 10 times the distance: $0.03\ \text{s} \times 100 = 3\ \text{s}$.

Step 3. 1 mm. 200 times the distance: $0.03\ \text{s} \times 40\,000 = 1200\ \text{s}$, which is 20 minutes.