DISCLAIMER- There is a method I've seen on various kindle boards but you need to take care when using a method like this because potentially it could mess up your kindle! So perhaps do some more research first before trying! I haven't tried it personally because I don't want to disable my screensaver - I'm just letting you know that this method does exist and that it's something that can be typed straight into your Kindle search box.
on google look up: ;debugOn ~disableScreensaver
or view the whole of this thread:
https://www.mobileread.com/forums/showthread.php?t=99229