Rule of thumb for the 737-800 is about 800'
pr hour. So, if you are missing 300', you are looking at 20-25 minutes.
Before climbing above optimum, that for specific fuel consumption, you are better off 1000' below optimum than 1000' above. If you are below the tropopause, your true airspeed will also decrease for a given mach number. It's not always worth it...