Shout Duration Statistics

In addition to botting (Instagress), I've been buying 24hr shouts from multiple pages for my company page for about a year now. I'm happy with the results.

I want to know the best cost/benefit shout duration. Are there any statistics concerning the duration of shouts/posts (or post half life)?

The first 15 minutes are the most important for exposure, I don't buy shoutouts, but it I were to I wouldn't buy more than 1-3hrs worth