How long is it before a vasectomy becomes permanant?

Here ... Precaution against pregnancy after vasectomy should continue until pos-vas semen exam shows no sperm. Generally, some 85% of men will have no sperm in their semen some 2 months or 15 ejaculations after vasectomy. So, it becomes permanent after no more sperm in semen; but, occasionally, post-vasectomy vas-reconnection may happen in.