Best is earlier --- it will primarilty depend on how well you can sleep w/ it. If its affecting the quality of your sleep, whatever value you get from the fatburning effects will be lost to shitty recovery.
I'd do it first thing in the morning, see how long you notice before it wears off & then reload. But do it before 3 pm. I think general in practice experience says to not pop thermos after 3-ish pm because then it will affect your sleep. You can experiment to find out what your tolerance is but those are the guidelines.