It definitely takes a long time to build something that would be successful and especially complete. I can't really say how long I took to build out either of my games Eliyo or Oripets (still in progress) since it's not like I worked only on them having a full time job in addition, but I would say at least months even if dedicated full time.
Some things can be pretty quick. For instance I can get a site going with a working registration system using Laravel in a few minutes. But, if I then decide to customize it in anyway, that takes more time. For Oripets for instance I've recently spent a couple days time working on a update to add in a feature so younger kids can sign up with parental permission. That required an update to add in age for users, age on the forms, new area for child registration that is different handling than adult registration to just name a few things.
Creating pets associated with a user is pretty simple too, but you usually need a little more than that. For instance even with the adoption system in the case of Oripets you select a pet species, color, stats, hobbies, preferences etc.. It's a relatively simple system, but it takes time to design and decide all these things. And that doesn't include any actual features of playing with or caring for the pet in anyway.
Even a simple system will take some time to setup, but I think realistically things need to be more than just basic.
So overall I would say there is a lot that goes in to it, and it can take a long time. But how long will depend on exactly what you are building and also your skill as a developer. Even for myself I have gotten a lot faster with Oripets than when I started out with Eliyo.
