A couple weeks ago I came up with the core movement mechanic for a new game design. I spent some time making rough prototype components and trying to figure out what direction to take this design and what other mechanics to mix in with it. I've basically not had any solid ideas until today when I came up with an obstacle mechanic that would make a good compliment to the movement mechanic. Now I'm feeling like this design is starting to get some traction but I know I'm not out of the design woods yet.
Anyway, based in this experience I wanted to ask how long before game ideas really solidy for other designers.
Yes the word 'solidify' is a little vague in this question. I guess I'm using it to mean when the design goes from a rough concept to when it really starts to take shape and have a definate feel which resembles what you want in the finished product (even in prototype form).