Let's look at this script:
If we hide the mouse, the biggest score we'll get is 1!
So we want it to re-spawn.
Right click the piece of script that makes the mouse spawn somewhere random, click duplicate and replace the block saying "hide" in the above script, with this new script!
Great! Now the cat spawns somewhere new after the cat eats it!
Now, we want a time limit.
Make a new variable called "Time", and in we want the time to change, so in the backdrop part, put "When the green flag is clicked, set 'Time' to 0"
Then, interpret my English into a script that does this : "Repeatedly for 10 times, wait one second, and change the variable 'Time' by one.
And then, in the control section, use a script that says "Stop All".
This script will stop all the scripts in the entire program.
If you need any help, please ask in the comments. :)