Yes a bit of a curse, the cost of free.
If you have just reinstalled avast recently, then the nag screens are a little more frequent, but should taper off.
I have been using avast free from late 2003 and whilst this nagging has become more frequent in the last couple of years, I don't get so many as to raise my blood pressure. I guess I have a higher pain/nag threshold.
I don't know why marketing think that this approach is productive, I feel that it is counter productive, turning down the volume/frequency would possibly yield better results. It certainly couldn't be any worse, imho.