How long does it take for sperm to rebuild in a 30 year old man?
If everything is working the way it is supposed to, it is on a daily basis. 100 plus million a day, like clockwork.
A man will always keep on producing sperm and therefore will always be able to make a fertile woman pregnant, His sperm may become less reliable and of a lower quality though. So long as a woman has not gone through menopause and her periods have not stopped then she can get pregnant to any man that has viable sperm.