How would one convert 5 decades to months?

By multiplying by converstion factors. 1 decade = 10 years, and 1 year = 12 months. If it helps, you can set up a line and put dimensions which are equal to each other on top/bottom of the line. The line acts like the line in a fraction.

10 years | 12 months | 5 decade |

--------------------------------------------- = 600 months

1 decade | 1 year