At the longest
Definition 1 of 1
Pro Tip 1/3Maximum Duration
Use it to specify the maximum amount of time expected for an event or action to conclude. 
Your order will arrive in three days, at the longest.
Pro Tip 2/3Expectation Setting
It sets expectations, indicating that something could be completed sooner, but definitely not later. 
I'll read the report by this afternoon, at the longest.
Pro Tip 3/3Emphasizing Speed
Often used to emphasize quickness or efficiency, suggesting a task won't take much time. 
The meeting will last 30 minutes, at the longest.