Hey Dudley :)
Jenn here from Shopify!
You're right that the built in abandoned cart feature we offer gives a 6 or 24 hr option and that's for two reasons - we found they were the two times with the best conversion, and also it's a good plan to not extend beyond 24 hours in the event you're only sending out one email.
If you want to check out more about best practices for times when using multi-email options like with apps, the fine folks at Salesforce ExactTarget wrote this cool article --> http://www.exacttarget.com/blog/13-best-practices-for-email-cart-abandonment-programs/
You've likely had a look already also at these great email ideas for abandoned carts, too... but in case you haven't, take a look! https://www.shopify.ca/blog/12522201-13-amazing-abandoned-cart-emails-and-what-you-can-learn-from-them
Hope that helps!
- Jenn
