You can never have too much....
Since I am becoming fairly active with my printing I am using a lot of filament. I am experimenting with HIPS since it is quite a bit less expensive than ABS. (HIPS is $25 per 2.2lbs and ABS is $42 per 2.2lbs) ABS is nice because I can use commercial glue/solvent for combining pieces. After quite a bit of looking I have found some ABS filament that is a bit lower in price ($32 per 2.2lbs) but still has decent quality.
I also try to have quite a few different colors on hand for different projects. Grey and Black are my initial goto colors.
When I am actively working on projects I use up a spool every 1-2 weeks.