I think one month free trial of the paid versions of avast is ample enough time for testing to make a decision on wheather you would like to pay for the product, once installed the benefits are noticeable immediately, avasts good will come’s from providing such a feature rich free version.
Not to long ago avast did run a promotion where you could win a free license but that has since ended so next time they run a promotion maybe you will be able benefit from it if you get in quick.