Hi Florin,
Understand your attention, but avast in last few months often bring the promotion to purchase let e.g Avast Internet Security for 2 years and then got 1 year free.
I think each company have their own strategy, meanwhile avast has provide avast Free if the customer required the quality for a long time period before purchase.
But for paid version, i think for 30 days is enough already.
cheers,