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?