It depends greatly on species, cultivar, and location. @fruitnut has written extensively on the seriousness of providing high-chill hours to peaches listed as such. @applenut broke the record books by growing what were thought to be high-chill apples in a low chill area (zone 9a, Riverside CA). Many people will vouch that high-chill sweet cherries will not produce much in a “low chill” (zones 9 and up) areas. I’m in zone 10b and have success with some Prunus that are rated in the 200-350 range.
Chill hours are an estimate. The metric was originally developed in zone 7 and below (places with 10-year average winter lows in the teens … 10F to 20F). The actual chill hour label comes from testing a cultivar for 5 years and then deciding if it works there.
The extension to zones 8, 9, and 10 has been problematic. In fact there are at least 3 methods for measuring chill in those zones. As @alan recently pointed out, cloud cover in these zones can make a huge difference. It would seem that “more cover” → “more chill”. So I think there’s more to estimating fruiting vigor than temperature.
This past year I started measuring chill hours at my site. Two standard methods have come up with 41 hours so far, while the Utah method has come up with -391.5 (negative). And yet, I’m certain that bumper crops of many fruits are coming.
@Isaac-1: your profile states you are in zone 8. I recommend that you view “chill hours” with caution – i.e., a bit of folklore that sometimes is correct for some species cultivars.