To extend the life of your wood deck you should seal the wood deck every 6 months to a year.
Some people go a couple years without sealing their wood deck and that is okay as well but do not go longer than a couple years without sealing your wood deck if you want the wood deck to last a long time.
Most wood decks that are properly stained and sealed can last from 15 to 20 years before they rot out and need replaced.
If you want to make a deck last basically forever then it's best to make your wood deck using composite decking boards.
The composite decking boards can basically last forever and never need stained or sealed and they also look very nice as well.