Only Avast can say when support for Windows 7 will occur.
We users can only take notice of the continued support for winXP many, many years after Microsoft dropped support. Official support for XP/Vista in the form of Avast has only officially ended this year, the last program version for XP/Vista is avast 18.8 see,
https://forum.avast.com/index.php?topic=220639.0 to get some idea of just how long after Microsoft drop support.
Even then XP/Vista users on 18.8 can still get virus definition updates, so there is still limited support after that deadline. Mentioned in the topic link that I gave, there may also be the possibility of some future program updates for avast 18 (the last supported program version).
So, based on the above, the end of support for windows 7 by avast is likely to be some reasonable time in the future.