Depends on how well insulated the garage is and how cold it is and how warm you want it to be.
The stoker in my well insulated 24 x 20 workshop keeps it at 60 overnight just running on the timer settings of 2 minutes on and 11 minutes off in the coldest weather. If I am in and out with a lot of air turnover then the 'stat will kick it on between timer calls.
If the garage isn't well insulated and its real cold out I could see it working because the 'stat will give enough heat calls to keep it lit. You may need to find out through trial and error what outside temp causes an outfire and then go back to the current way you are running it when it hits those temps.
The alternative is to wire in a parallel timer circuit.....or program a timer into an arduino with a 'stat input and some relays....