Sun takes ages, until fully deleted. I tried it once for fun and it didn't even flip a bit after 2 days or so during summer at like 500m elevation.
Silicon needs like 4.6eV or more to get it's electrons removed, which equates to 270nm or less.
Due to atmosphere, there is very little light below 350nm, so very little light capable of erasing EPROMs.
Maybe it works a little better if you live like on top of a mountain, but any cheap UV disinfection lamp with starter electronics from broken fluorescent lamp does it like a 1000 times faster.