Unfortunately it's not possible.
Straight from the horse's mouth:
Offline areas that you downloaded on your phone or tablet need to be
updated at least every 30 days.
If your offline area expires in 15 days or less, Google Maps will try
to update your area when you're connected to the Internet and plugged
into a charger.
(from Google Maps Help - Download areas and navigate offline)
There's a bit of conjecture online about what the reason for this may be, and whether it's potentially related to Google being worried that maps will be out of date. On the same link above, I would say this gives a clue:
Note: Downloading offline areas isn't available in some regions
because of contractual limitations, language support, address formats,
or other reasons.
Although there's no official word from Google on it, I'd say it's highly likely that this time limit is related to contractual limitations at least in some areas. Kindof like how iTunes rentals expire, or Spotify Premium offline playlists expire: if there was no expiry date at all, then there's no difference between 'accessing' and effectively 'permanently buying for free/cheap'.
If offline maps are very important to you, you may want to consider apps that support OpenStreetMap (or see this answer).