Many ship with a standard image or set of images displayed when powered off or suspended. Presumably those display for long periods of time. My experience over ~3 years with a similar Onyx BOOX device has been that those images don't burn in at all.
What is experienced is temporary ghosting which varies by display mode (these can be set either globally or per application), when partial refreshes are made. The solution is to do a full refresh every so often, for which there are a number of built-in settings, or which can be triggered manually on the tablet.
For either the author here or others looking to implement similar projects, you can very likely safely skip any consideration of burn-in, though if your interest is in crystal-clear display, full refresh can be used. Particularly when display updates are infrequent, say, < 1/minute, or any longer period.