Practically if one washed and did not say the blessing, until when may the blessing be said?
The blessing may be recited until he Davens Shacharis.[1] Practically it is best to first use the bathroom or touch a covered area, and then rewash with a blessing.[2]
[1] As rules the opinion in Admur 6:2; Shulchan Hatahor 4; Ketzos Hashulchan 2:4; Regarding saying the blessing after Shacharis-see Halacha 2 Q&A.
Other Opinions: The Shaareiy Teshuvah 4:4 rules that if an interval took place one is to wash without a blessing as the impurity has already been removed and hence the blessing has been lost. The Ketzos Hashulchan 2 footnote 16 argues on this ruling and so questions the Birchas Habayis 32:5; Minchas Elazar 2:44.
[2] See Biur Halacha 4 “Afilu”; Ketzos Hashulchan 12 footnote 9