I bought a prenatal vitamin from the local Chemist, one of the health expert sugguested me to. Now I am wondering whati should do with it? I just received my periods today. How long should I take this prenatal vitamin for?

Take it every day just like you would a regular vitamin. Swallow it of course.

I am 44, with 4 children, no more children in my future, and I STILL take prenatal vitamins.

Pumpkinjojo... it is best to already be taking the prenatal BEFORE you become pregnant. Besides the obvious boost in nutrients that you may not always get if your diet is not the best, folic acid is the main reason to be on them before conception. Having enough folic acid (sometimes you will see it as folate or B9), can help prevent certain birth defects that occur very early in a pregnancy, often before many women even know they are pregnant. It is important to have enough folic acid throughout your pregnancy as well. I know some locations focus mostly on the first 12 weeks, but folic acid is also important for other functions of the baby during pregnancy. In the third trimester, for example, there is an increase in brain development and having a good amount of folic acid is important in that regard as well.
Like J_9 said, you can take it like any other multivitamin.