Transformer Overload Capacity

It depends on a lot of things, such as ambient temperature, load cycle, initial loading, type of insulation, size of transformer, method of cooling ie forced or natural, how much life expectancy you want to sacrifice etc. As a first pass have a look for the specification the transformer was built to, or ask the manufacturer. GE Power produce a handy little guide called the Distribution Transformer Manual GET-2485R which has lots of information on overloading transformers. I'm not sure if it's available electronically though.
Regards
Marmite

ANSI/IEEE Std C57.91, IEEE Guide for Loading Mineral-Oil-Immersed Transformers

You can find requirements that will allow you to provide protection up to 250% of the transformer's FLA.

At one time, I had a pamphlet from ABB (I think) regarding their recommendations for protection of their transformers. It showed the short-time overload ratings near that value, but for only several minutes or so. My suggestion is to find out your transformer manufacturer's recommendations.

Unless you have overtemp protection, your overload protection should be much closer to the FLA unless you want to void any manufacturer's warranty or disregard their recommendations.