How long do USB power banks usually hold a charge?

This question is missing some info. What exactly do you mean by hold charge? Some people have asked me similar question but it turned out that they actually wanted to know how many times a particular power bank could recharge their phones before needing a recharge itself.

Lifespan of power banks

If you mean idle time, do you then mean never used at all or used intermittently? Either way, there are many factors to consider. See Solar Power Bank Quality Assurance: Power Bank phone Chargers

Battery power degrades at a constant (but immaterial) rate over time. I have a HG solar power bank still in the original pack that I have not used. Its been in my wardrobe for 3 years and I still have over 50% power in it - as at 2 months ago when I last checked it. Does this answer your question?

In summary, many factors affect power banks such as authenticity, temperature, climatic conditions, lifecycle, and a handful of other stuff. 

Hope this is helpful.

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