ISO and the final image output is going to vary a lot, with such variables as total scene illumination, RAW vs JPG (or whatever format your camera will output), overall color cast of the scene, etc.
Short version, testing is the best and easiest answer to figure out how YOUR camera does at high ISO’s. Set your camera up for your normal shooting settings (ie image format, corrections, etc) with a static scene, static lighting, and static aperture (preferably a mid-range one like f/16). Make sure the lighting is low enough you can get to your max ISO and start shooting working down in 1/3 or 1/2 stops. Needless to say, do this off a tripod. I recommend using a mirror lock up shutter release if on a DSLR, and regardless of camera type a remote release to trip the shutter to minimize camera shake as the exposure times get longer and longer. Have a nice contrast target for the autofocus and verify that it is keying off that target each time so the focus is consistent.
Here is the real important part unless you are just geeking out about the science and technology.
On the device you are normally going to using to show your photos (for most of us a smart phone or tablet), have a third party look through the images. Starting at lowest ISO (so in reverse order shot), ask them to go through the photos, and to tell you when they notice the difference. Don’t tell them what they are looking for or what you are testing, just say “the difference.” You are going to realize real quick what most people notice is about 5x-10x worse than you will. They are going to see phantom color shifts, focal changes, etc. A whole bunch of things which didn’t occur. With modern camera’s it’ll probably be in the 10K range before people start commenting on grain. Ask them which pictures they feel are unusable, you may well be in the 25K range, depending on their artistic tastes. If viewed on a cell phone it may well go higher.
Pixel peeping on a 36" monitor can be fun, but you are going to drastically limit yourself on what you consider “acceptable” if you do that.
If you are consistently at the same location (I’m at Circuit of the Americas a lot) or same general environments (graduation photos in the Texas summer) you can run the same test on site to see if you have any changes.