There's folks who're recreating stuff like Helm's Deep or Minas Tirith or Notre Dame in game and those builds look awesome. You probably can spend the rest of your life in Valheim, building stuff, as long as you don't run out of inspiration. I got on and off phases. Just spent a couple hours in game decorating my newest house. Took me about six hours to build it (and it's mainly wood and some core wood - huge stone buildings like, e.g. a castle, take a lot longer). Could probably easily spend as much time on decorating everything so it actually looks lived in (finished the kitchen today, took a lot of playing around with item stands but in the end it worked out).

You can use stuff for things other than its intended purpose (e.g. round wood shields as plates, wooden tower shields as platters, mushrooms or glowing trophies as lighting, etc.) - just use your imagination and try. Or watch a ton of building videos on Youtube to see how other people do it. Maybe indeed start off by watching some building videos on Youtube to see if you like it.

If you're playing solo, you can also enter creative mode via the command console, which allows you to build without material costs (read: build everything for free), if you just wanna build and don't care too much for the rest.