Calculating run time for a device with variable current draw
Your calculations are slightly off, as the duration you averaged over was 3 seconds, not (2 + 0.5):
(2 * 2 + 20 * 0.5) / (2 + 0.5)
 = 5.6 mA average current draw

2400 / 5.6 = ~ 428.6 hours
 = 17.86 days running time

This is a good enough method of estimating run time. 

Actual results will vary due to battery condition, charge status, temperature changes, and of course whether your current measurements are reliable and repeatable.