This script reminds you when someone hasn't replied to an email after a certain period of time.
To be reminded of an email, just add a label "Wait X days" or "Wait X weeks", * where X is how long you want to wait for a reply. You can also copy yourself on the email (To, CC, or BCC) like mark+1d@gmail.com and the next time processSentMessages from gmail-no-response.js runs, it will add the appropriate tag ("Wait 1 day" in this case). That's it!
Visit the Google Scripts site and create your first script. Scripts are stored in Google Drive, so you can edit them there later on.
Then, get the Gmail - No Response script and paste it into your new script document.
Next, click the clock icon to create a trigger that will run this script on a schedule. main and processSentMessages should both run at least daily. I set mine up to run daily at midnight.
H/t to Jonathan Kim for the original script.