Keep in mind that these things have duty cycle limits for a reason. I suppose you could get one that would run for hours on end - but it would probably cost over $300.00
When you are shopping you need to be savvy, watching product description and looking for technical information. There are a lot of cheap jewelry cleaners out there described as "ultrasonic" - but they really aren't. There is a huge difference in the number of vibrations per second a real ultrasonic puts out versus a jewelry cleaner. (Actually most jewelry would be trashed by a real ultrasonic.)
Just be aware of what you are buying - that's all. If you want to steep some liquid a "sonic" jewelry cleaner will probably suffice, but if you want to clean gear you need a real ultrasonic. (And no - you can't get a real ultrasonic for 20 bucks on eBay. Sorry.)