Launch script after command finishes
There's no post-execution hook, only a pre-execution hook: if set, the value of shell variable PROMPT_COMMAND is executed prior to issuing the prompt. See man bash for details. 

An option would be to create a function that tells Terminal to beep after the long command is run, so you know when it finishes. Such a function would be:

$ function r { $@; beep; yourscript; }


Your script will be automatically launched after Terminal makes a beep.

Use the function to run those long commands like this:

$ r longcommand


You can add the function to your .bashrc file to avoid defining it every time.

If you're not a fan of audible bells you can tell Terminal to flash instead: