I think you're wrong there. Pollen doesn't get stronger the longer you store it. Thats a little counter-intuitive don't you think? Pollen is the plant equivalent to human sperm, made of one vegitative cell that splits when it fertilizes a female plant. http://en.wikipedia.org/wiki/Pollen
Now, I can't give you an estimate on how long pollen lasts, but I would recommend you keep it in a dry, dark place and use it soon.

Happy growing! :thumbsup: