If you've heard of the pomodoro technique, consists in working non-stop for 25 minutes, taking a break of 5, and a break of 20 every 4 loops. Some apps start the timer and switch one another instantly. And let's face it, you can't switch from working to a break in discrete time. This snippet starts a timer for 25min and let's you know when it's over. Then you take your brake, get up, get a snack an time it with your watch, then come back and start the clock again.
alias pomodorostart='echo "Pomodoro start at $(date +%T)" && sleep 1500 && notify-send "5 min break"'
1500 is the number of seconds to time. If you want to use it for the break too, just change it to
300 and have like
alias pomodorobreak='echo "Pomodoro break at $(date +%T)" && sleep 300 && notify-send "Get back to work"'