Below comes from the Letrazol fact sheet
In primary breast cancer, if letrozole is your only treatment itís usually taken for five years. If letrozole is given after five years of tamoxifen, it will usually be given for two to three years. However, clinical trials exploring the optimum length of time to take letrozole are ongoing.
If letrozole is being used to shrink your cancer before surgery, your specialist will advise you how long you can expect to take it for. After taking it for approximately three months you will usually be offered tests to see how the cancer has responded to the letrozole.
If you are taking letrozole for breast cancer that has come back in the breast or surrounding area, or for secondary breast cancer, you will usually continue to take it for as long as it is keeping your cancer under control. Your specialist will discuss with you how long you need to carry on taking it.
The Bottom line here is to talk to your oncologist. Nothing is in black and white.