Pretérito imperfeito translates to: We haven't seen each other for twenty years. (accepted by DL).
We haven't seen each other for twenty years.
Nós não nos vemos há vinte anos.
We hadn't seen each other for twenty years.
Nós não nos víamos há vinte anos.
Use of pluperfect in English becomes imperfect in Portuguese ??
The verbs should be in agreement as far as time is concerned, as in:
"Nós não nos víamos HAVIA vinte anos." if it is passado imperfeito or
"Nós não nos vimos há vinte anos." for the passado pretérito.
It had been 20 years since we had last seen each other.