Is it bad for an RF amplifier to leave it on with no input?
These amplifiers (especially from minicircuits) are definitely designed for 24/7 usage throughout years. So, as long as you cool it sufficient (and radiation might be enough, considering it comes in a package including a large heatsink), you should be fine for decades.

If you can control the signal going in: simply don't put in any signal, but e.g. terminate the input with a 50Ω terminator¹. That will minimize power consumption and thus heatup.


the terminator of the passive component type, not of the Sarah Connor type.