I'm not a fan of "long tones." Hear me out (I do have a degree in trumpet performance):
As trumpet players, we hardly ever just play a single note and hold it as long as we can. I can't even think of one instance off the top of my head, with the exception of the intro to the overture "Rienzi," but, even then, we don't just "hold" it, but rather crescendo and decrescendo in a controlled manner.
I subscribe more to the Cichowicz idea of "moving long tones." If I have only 30 minutes to practice in a day, I'll do some Stamp exercises and then maybe a lyrical etude. The goal is to develop sound, centering of pitches and continuity of air, something that long tones doesn't help much with.