The BEST thing to do is to make it immediately and everything remains simple.
If one doesn't there are some Sefardi poskim who say that you have a half an hour.
Rav Ovadiah Yoseph [יביע אומר ח"ח סי' כב וחזר בו ממש"כ בחלק א] found a Ritva in Maseches Pesachim [מו] who implies that one has 72 minutes and so he ruled.
I cannot describe fully the "rischa di'oraisa" - the fire of Torah, that was spawned by Rav Ovadiah's understanding - or as many would have it "misunderstanding" - of the Ritva. There is a sefer called "Birkas Eitan" on the bracha of Asher Yatzar [654 pages!!] that details the intellectual brawl that broke out. Whew, those rabbis take their Torah seriously...
Bottom line - if you are Ashkenazi you may say the bracha until the next time you feel a need to go in line with the ruling of the Mishna Brura. If you are Sefardi then what you should do is ..... No way! I am NOT getting involved in this fight [although as usual I have an opinion:-)].
Ask your local Chacham. Chazak Ubaaaaruch! Yishtabaaaachhhhh Shemoooooo laaa'aaaad! I love you guys:-)
And PLEASE peruse this [I recently misused the word peruse to mean to take a cursory glance but was informed by R' Yonatan Shai Freedman that the word means to read thoroughly. Thanks to him and anyone else who corrects me].
