This is actually quite common among people who have mechanical valves. When you first start on warfarin, you are still functioning at quite a low level. You are not exercising, eating like normal etc. As you begin to do these things, your heart rate increases, your blood is pumped through your liver more efficiently and you start taking in more vitamin K with more food. All of these lead to needing higher warfarin doses. It happens so regularly that it is almost a marker that you are getting well.
If it gets to the point where you are taking four times as much as when you started, it is time to look for a cause. This could be interacting drugs, particularly things like Fiorinal, Tegretol, Bellergal, vitamin K containing vitamins and possibly some natural products.
There is no true tolerance, just things that interact, or better health.