Studies of Internet usage -- Pew Research is a recommended source -- have shown roughly 40 minutes/day of primary social media (FB) use.
Divide that 40 minutes by messages, and you get the time consideration for each message. 10 messages = 4 minutes each. 100 messages = 24 seconds each. More time on one message is less, or none, on the others.
I've found a few citations for daily information consumption by experts, which are interesting.