Jumping in and running NixOS on my desktops at work and home is how I really learned it. I'd say that aside from routine usage, several hours of doing something at least a little 'custom' with it (packaging something new, defining project environments, writing or modifying NixOS modules, writing your first override, packaging for a new language ecosystem, etc.) per week will have you feeling pretty comfortable after just a few months.
Basic stuff is easy, though, and I'd say it only takes a week or two for administering a VPS or something like that to start feeling really good. Something about the iteration loop with nixos-rebuild is satisfying and somehow very freeing. So you'll get that early payoff in a few days or maybe a couple weeks. Then you'll hit a wall and some problem will seem way harder than the experience so far. But you'll figure it out, and after a few months of occasional walls like that, real difficulties will become pretty rare. In a year, you'll be addicted. :)
For me, the Nix learning curve was challenging, but it was not a grind. It has frequent moments of payoff that start pretty early.
I'd say my main advice for keeping it enjoyable is just this: don't hesitate to engage the community if at any point you feel stumped. Nix has a truly outstanding community in terms of both technical expertise and generosity. Seeking help will accelerate your learning journey a lot, and make it feel less effortful.