Nitrous oxide (N2O) is used as an anesthetic (laughing gas) and in aerosol cans to produce whipped cream. It is also a potent greenhouse gas. The decomposition of N2O is first order. It decomposes slowly to N2 and O2:

2N2O / rightarrow 2N2 + O2

How many half-lives will it take for the concentration of the N2O to reach 2 % of its original concentration?

The half life of any reaction is the amount of time it takes to consume half of the starting material.

In this scenario, the initial concentration is 100% and we are going to 2%.

100% - 50% : First Half life

50% - 25% : Second Half life

25% - 12.5% : Third Half life

12.5% - 6.25% : Fourth Half life

6.25% - 3.125% : Fifth Half life

3.125% - 1.5625% : Sixth Half life

This means it would take about 6 half lives to reach the desired concentration.