I disagree a little with the above poster.
I don't think it matters how long ago you were having problems, it is more about what your hair looks like now and your expectations. For instance, if you experienced hair loss 5 years ago but it stabilised and it's no worse, then you could achieve good results; on the flip-side if you experienced aggressive hair loss a year ago, where you are now a NW4 (unlikely obviously), then finasteride will probably not bring you back to NW0.
