Read Software Estimation by Steve McConnell and recommend it to your manager:
The person doing the work the one best suited to estimate it.
Also, start tracking estimated vs budgeted time in some searchable system.
Next time this comes up, look up how long it actually took to complete a similar task instead of thoughts and prayers.
If boss won’t track historical budget vs actual, track it yourself.
That pattern is also recommended in the book. Break down estimates into chunks of 5 hours or less.
With lots of smaller tasks, estimates tend to both be more accurate due to smaller scope, and some of the over/under inaccuracies will cancel out.