I posted a similar question without the flawed math: Can I trust in advertised 4 stops of image stabilization in a cheap kit lens? ...that was marked as a duplicate, so I'll repeat the main findings of me here, so that the information in the duplicate question doesn't get lost. Note: this answer doesn't even attempt to give an answer to the flawed math.
Firstly, I have found that IS doesn't help much at 1/5 s on a cheap 18-55mm kit lens (see later for 24-240mm $1000 lens results). You need 1/10 s or shorter exposure for IS to help. It seems that if I really try, I can hold the camera steadily at approximately 1 / (0.5 * 35mm-equivalent-focal-length), being somewhat better than the 1 / (35mm-equivalent-focal-length) rule. This was based on 10 test images (55mm focal length, 1.6x crop sensor) taken at 1/5 s with IS (mostly shaken), 1/10 s with IS (mostly fine), 1/20 s without IS (mostly shaken), or 1/40 s without IS (mostly fine).
So, at 55 mm focal length, for my test conditions, the IS helped only for two stops (advertised as four-stop IS). The 55 mm is the longest focal length on this 18-55mm zoom, so clearly, you (or well, I) can't trust the image stabilization ratings.
I also tested a 55-250mm lens @ 250mm and took some non-shaken pictures at 1/30 s shutter speed. If we believe the 1 / (35mm-equivalent-focal-length) rule, the shutter speed should be 1/400 s, and if we believe the slightly modified 1 / (0.5 * 35mm-equivalent-focal-length) rule, the shutter speed should be 1/200 s. Based on whichever rule we believe, the IS helps for at least 2.7 stops or 3.7 stops. I say "at least" because I didn't test longer exposures than 1/30 s.
I also took ten pictures using 24mm lens at 1/20 s exposure. They were mostly non-shaken, although one had very slight camera shake. If IS can't help at longer exposures than 1/10 s, on a 24mm crop lens, IS helps for one stop only. A Canon 24mm f/2.8 has twice the light collecting ability of Canon 18-55mm f/3.5-5.6 zoom which @24mm opens to f/4. So, on a 18-55mm lens used at 24mm, it's a much better idea to remove that lens and attach a 24mm f/2.8 prime.
Main findings, applicable for cheap crop sensor zoom lenses:
- Short focal length on a 18-55mm f/3.5-5.6 zoom: use a 24mm f/2.8 prime instead, no need for IS as the prime is equivalent unless you really need lots of depth of field, and the prime is better for fast-moving subjects, too.
- Long focal length on a 18-55mm f/3.5-5.6 zoom: use a nifty fifty (50mm f/1.8) instead, no need for IS as the nifty fifty collects 2x the amount of light at longest acceptable exposure time compared to the zoom, and it's much better for fast-moving subjects. However, if you really need lots of depth of field, a kit zoom with IS may be better.
- Short focal length on a 55-250mm f/4-5.6 zoom: use a nifty fifty instead, again.
- Long focal length on a 55-250mm f/4-5.6 zoom: now image stabilization helps a lot!
However, the above apparently applies only to cheap kit lenses. I bought a $1000 24-240mm Canon RF lens for EOS RP. It is using a much better "dynamic IS" technology that combines sensors in the camera and in the lens. It is advertised as 5 stops.
I took 10 pictures at 1.3 second exposure times @ 24mm. None of the pictures can be classified as genuinely shaken, although very slightest hints of camera shake can be seen in about 2 or 3 images. The very slight hints of shake are visible only if you view the image at 1:1 ratio on a 96 dpi computer screen, which most people don't.
If we believe the 1 / (35mm-equivalent-focal-length) rule, we get from 1/24 s to at least 1.3 s, which is at least 5 stops of improvement. If we believe the slightly modified 1 / (0.5 * 35mm-equivalent-focal-length) rule, we get from 1/12 s to at least 1.3 s, which is at least 4 stops of improvement.
So, it appears that on the cheapest kit lenses, the IS ratings are deceptive. But, on the more expensive lenses, the IS ratings can actually be true.