I remember, during college (2008), my web design professor made a point to keep all images' file size at/below 40K (didn't matter what the dimensions). I have made an effort to do so ever since, especially with mobile optimization being so important.
But now, with broadband and LTE/4G speed internet, larger data plans... and I being out of college for a while... are there any file size (not dimensions) standards out there?
Side note: My project is dealing with product photography. I'm looking at Amazon.com as a benchmark and note that their product images are under 40K (usually) with a zoom image under 100K (usually).