My games tend to take forever, and probably won't be finished due to me wanting them to be perfect. I usually spend an hour or so a day(sometimes only a few hours a week) working on mine, as I'm pretty busy most of the time.
If you have all of the ideas in your mind and on paper (digitally or otherwise), it usually doesn't take as long. However, most people change their ideas constantly, implement new ideas, or simply take advice from someone, which ends up taking up more time due to having to change things around or by adding things.
That about sums it up for me. I had everything worked out on paper, but I couldn't stick to it because I always found better ways to do something, or an idea that seemed better. I also recoded the class/character creation system around 5 times.
Originally it only had 3 party members with gender and race. I had 1 character per race/gender combination per party member(around 18 different characters). It was horribly inefficient, but I wasn't too experienced with event systems at that time. Then I changed it to 1 character per race(for each member), using classes for gender, resulting in around 9 characters. Still horrible and badly done. Took a break from the project, came back a year later and realized how bad it was and recoded as efficiently as possible, eventually upgrading it to 4 party members. 4 characters, a handful of classes and simply changing graphics and adding skills after character creation. All these changes alone took up a large chunk of time.
Honestly, though, I'll take as much time as I feel I need to make it.