Does a timed reaction "ready indicator" paper exist?
I'm not aware of any commercially available examples, but I know that the speed of a fluid wicking through a paper microfluidic device has been used for timing. I'm also reminded of pencil detonators, dating back to WWII, where the time taken to dissolve a lead wire is used for timing. Since electronic timing is now so cheap, easy, accurate, and reliable, I'm not surprised that chemical timers have pretty much died out. There are a number of chemical phenomena that could be exploited for timing, but unless you need a huge number of cheap, one time use timers with relatively poor accuracy, there are probably better choices.