I use different formulas for summer and winter, but i would say I finish a bottle foundation after about 12 months worth of use. More than once, I've had to buy new foundation because it went bad (about a year and half old) before i used it all.
How fast you would use up a foundation would depend on
1) Amount of foundation purchased (some of my bottles are 1 oz, some are more)
2) Application method (sponges waste more product since they are absorbent, as opposed to using a brush or your bare hands)
3) How often you wear foundation
4) If you wear foundation on your whole face (I often use foundation only on my cheeks, nose, and chin areas)
5) The type of foundation (my tinted moisturizer gets used up more often than my other foundations)
HTH