centre tap transformer 240V – 24V @ 250mA
Each half can still supply 250mA. So you 2 12V outputs @250mA.

A common way to use a center tap transformer is to provide postive and negative supplies with 4 diodes.

Be careful about using a bridge rectifier from each half as I see in some circuits, that won't work as some of the diodes short out others.

The total power output depends upon how the transformer was rated but it probably is good for 250mA DC output from each of the positive and negative supplies.