Not necessarily. Speaking for myself, sometimes it's all or nothing. I can't imagine having the same enthusiasm and persistence for a hobby than for a full-time effort, particularly if the latter involved burning a few bridges.
I'm not arguing it's rational, or that one should blow one's savings on it, but sometimes full-time is your only choice.
Development of even a simple game takes so much time I don't know how practical it is on 10 hours a week. Game studios have armies of employed people on 60+ hours a week each.
Game studios that employ armies of developers usually have tons of artists and spend ridiculous amount of time on graphics and assets. That's why a massively complex game like dwarffortress are never created. So there's tons of niches for complex games with ASCII graphics. (Sorry, pixel arts also takes ton of time)
As everyone else already said, control your scope or otherwise you never finish the game. Dwarffortress got where it is today by simply not aiming for great 3D graphics.
Also, Dwarf Fortress isn't done yet. It quite possibly ends with beautiful procedurally generated graphics (though I can't imagine that would happen with Tarn keeping it a solo operation.)
You can easily make a decent Flash game or experimental iPhone game part time. My first Flash game, with over 200 million plays, was made in two weeks including the time it took to learn Flash.
The additional hours will get eaten up by business stuff and maintenance once you're full time anyway.
I dogfooded my own advice, for what it's worth.
http://www.miniclip.com/games/stunt-pilot/en/
Two people, two weeks, normal working hours (Coffee Hero was an absurd crunch).
You may or may not have heard of the 48-hour game development event GameJam? http://globalgamejam.org It's possible to create something playable even in that time.
Also, Minecraft was originally developed part-time, according to Wikipedia.
Working on something serious part-time is hard, and I'm impressed by people who manage it. But it's doable.
Love it.
That way you don't burn through your life savings, and you're very much aware of the day to day. Bonus points if you've had failures in between your successes. Patrick is dead on here - it's not all roses.
If you can't succeed doing it for 10-20 hours a week as a hobby, it's doubtful working even 80 hours will work. Game development is sort of like being a musician. If nobody likes your songs, you can't make up for it in volume.
Game development is an irrational choice in terms of return on investment. Patrick is also dead on here. Even the successful developers would have made an order of magnitude more money if they had applied themselves equally and had the same luck in other industries.
You have to genuinely find the prospect of working on your craft full time more important than financial returns. And having the perseverance to hone your skills and produce successful games in your spare time, even if it takes years, is a good litmus test for that.
(I'm awkwardly referring to patio11 by his name even though I was replying to him to differentiate the you [him] from the you [in general]).