I think I received this message because too many time-dependent workflows were scheduled.
I did a research and what I can find is :
Salesforce limits the number of time triggers an organization can execute per hour. If an organization exceeds the limits for its Edition, Salesforce defers the execution of the additional time triggers to the next hour. For example, if an Unlimited Edition organization has 1,200 time triggers scheduled to execute between 4:00 PM and 5:00 PM, Salesforce processes 1,000 time triggers between 4:00 PM and 5:00 PM and the remaining 200 time triggers between 5:00 PM and 6:00 PM.
My questions is if an Unlimited Edition instance has 3,000 time triggers scheduled to execute between 4:00 PM and 5:00 PM,
Would Salesforce process my 3000 time triggers from
4:00 PM to 5:00 PM first 1000
5:00 PM to 6:00 PM second 1000
6:00 PM to 7:00 PM third 1000
Or would Salesforce only defer the second 1000 and erase everything after.