Use this script to trim all selected layers with the duration you specified, you can also specify the gap you want between the layers ( in frames ). You can also use this script for One selected layer also..
Click here to download this script Cut2Specified.jsxbin
Click here to know how to install it in your PC how to
Click here to download this script Cut2Specified.jsxbin
Click here to know how to install it in your PC how to