Buffer to Buffer copy is probably the best test for measuring bandwidth, but it doesn't represent the best layout for accessing data that is logically two or three dimensional, so stick to textures for that.
There really isn't a good one-estimate-fits-all-hardware approach to knowing how long things should take. The difference alone between 720p and 1080p is 2.25x and the hardware disparity between a reasonable mobile GPU and a high end discrete GPU is > 10x. So depending on whether you're talking about a Titan X rendering at 720p or a mobile GPU rendering at 1080p you could be talking about a 20x difference in GPU time.
I have a pretty good idea for how long typical tasks should take on Xbox One at common resolutions (720p, 900, 1080p), but that just comes from years of looking at PIX captures of AAA titles from the best developers. If you asked me how long X should take on hardware Y at resolution Z I'd probably start with the numbers I know from Xbox One, divide them by however much faster I think the hardware is and then multiply up by the increased pixel count.
It doesn't hurt to try figure out how close you might be coming to various hardware limits a GPU might have; just to see if you're approaching any of those. Metrics like Vertices/second, fill-rate, texture fetches, bandwidth, TFLOPS etc are all readily available for AMD/NVIDIA cards. The only tricky one to work out is how many floating-point operations your shader might cost per pixel/thread as you don't always have access to the raw GPU instructions (at least on NVIDIA cards you don't), you can approximate it from the DXBC though.