There actually are some good rules of thumb. 2 hours is generally the limit at which your insulin will be spike from a basic meal. As others have said, this time will vary depending on the food you ate. Some carbs, if they are really the slowest digesting types, will likely not spike your insulin in the first place, just elevate it. It could take more than 2 hours to drop, but it shouldn't take much more than that.
The real question here is why you think you only "burn fat" when your insulin is not spiked. It's really so much more complicated than that and you need to worry about the bigger picture.