A wood deck that is built properly and up to code and kept stained and sealed should last from 10 to 20 years.
After 20 years though the wood deck will likely need replaced regardless of keeping it stained or sealed.
You can build a deck using on wood boards which would make the deck last likely a lifetime but if you get 10 to 20 years out of a wood deck that's a good lifespan of a wood deck.
To preserve your deck use treated wood deck boards and stain and seal the deck every few years or so.
You could leave your deck untreated even if you've used non treated deck boards.
However if you don't treat your deck with a stain or other deck treatment then your deck could rot out much sooner and then you'd have to replace the deck again.
If you have just built the deck and cannot afford to stain the deck or treat the deck right away then that's okay but you should stain or treat the deck with some water proofing water seal to help the water shed off the deck so that it doesn't rot out.
You don't want to spend a lot of money building a nice new deck and then have it rot 5 or 10 years later and then have to replace the deck again.
I would recommend staining and sealing the deck with water seal every few years to preserve the deck.