Gnome Shell Extension notify you for how long you been setting since you activate the toggle timer extensions so you can take a break of whatever you are doing or start working on other stuff.

By Default Pomodoro extensions change number of cycle every 25 minutes and the cycle start counting by one.

How To install Pomodoro extension Toggle Timer For Gnome Shell on Ubuntu 11.04 Natty and Fedora 14 Lovelock

Download Extension files form Here.

Then copy the files to this path within the directory

Then restart Gnome Shell. Alt+F2 then type “r” without quotes.

Then it will appear on your gnome shell panel click on it to activate Toggle Timer.

Modify Time Cycle

By default each time cycle is 25 minutes, but if you want to each time cycle modify extension.js file.

Open extension.js with any text editor, I modified it using nano text editor.

then search for using “Ctrl+W”

Here I change time cycle to be 45 minutes instead of 25 minutes.

Change time cycle as you wish, then save the file and restart gnome shell again.

Thanks to codito (Arun)

Source.