Although we don't pasken like this, the opinion of the Shulchan Aruch is that one who relieves himself and hasn't yet made a bracha Asher Yatzar, and subsequently relieves himself again, must say Asher Yatzar twice. The Shulchan Aruch seems to hold that there is no time limit.
Would the Shulchan Aruch require someone who becomes religious to make an approximation (lower bound, to get around ספק ברכות) of how many times in his adulthood he relieved himself, and make the equivalent Asher Yatzar?
The Mamar Mordechai (s.k. 3) understands the Shulchan Aruch's position is that the second recitation covers all the times he forgot, so one would never say more than two. However, the Pri Megadim (M.Z. s.k. 2) disagrees and writes that according to the Shulchan Aruch one could recite more than two brachos. Rabbi Shlomo Zalman Aurbach addressed the issue of baalei teshuva. He is quoted as saying (Siach Halacha footnote 9) that even according to the Pri Megadim's opinion a baal teshuva does not have to say hundreds of brachos because 'asher yatzar' is a bracha of thanks and is therefore only said for the incidents one remembers specifically and therefore currently feels (at least theoretically, I guess) more thankful for.