@,
First of all, thank you. Secondly, I actually don't have to frequent art stores very often. I usually purchase Prismacolor pencils in their largest set: 120. I never actually buy markers except to outline everything I do, and even then, I use one black marker every five months or so.
My current set of Prismacolor pencils has lasted me about...three years now? I have yet to use an entire pencil out of it; in fact, more than half of the pencils haven't been used.
So no, I usually don't have to go and buy a lot of materials, but that's just me. Depending on the person and how much work they put into a drawing (ie coloring, backgrounds, shading, etc), their use of art supplies will vary. For me, I use each supply efficiently, but not to the point to where I run out quickly.