Haha I didn't even notice the suggested time frame. I just know that my internal veteran teacher time clock immediately registered it as 'multi-day'.
I would like to share a website my superstar CS colleague shared with me that she used at Cal when they did the exact same type of prototyping exercise. It's called marvelapp.com and is really easy-ish to use. You are allowed two free projects.
If you would like to see the Txt Ur Grndkdz paper prototype as an electronic prototype, here is the link:
It doesn't have the same bang as seeing it in your phone (which you can, if as the creator you want to send an SMS link to yourself), but it's pretty cool. I just used a snipping tool to clip the UI screens from the Lesson 4 Activity guide (which is why the first one looks so awful) and then uploaded them to the site. Took under 10 minutes.
There is an app version for ios and Android. I tried the ios version and it was a bit glitchy, which is why I switched to the web version. You might want to try it in your device to see if you can get it to work better for you.
I embedded the app on my school website and am going to have the kids play with it. Then later I'm going to have the kids make an e-prototype for this project from their paper screens, letting them decide to use a phone or computer. We'll see how it goes!