It depends on the quilt, and my processes may differ.
Sometimes I work through it when I'm in the LQS looking at fabrics and I can mesh most of it out there, then come home and finalize what I want to do.
If it's a fairly simple quilt, or a "free form" quilt (like a landscape) I'll just jump in and see what happens.
If it's a more complicated quilt that requires math, I use EQ.
the last quilt I designed on EQ took me close to a year to finalize the design (including colors - which changed throughout the project). I finally started the piecing in January and I have a LONG way to go yet. It's DJ with a modified setting.