Your breasts don't "flip a switch" at some arbitrary point and start producing hindmilk instead of foremilk. Instead, think of the beginning of a nursing session as being like turning on a hot water faucet.
The first water you get out of the tap isn't usually hot, but cold. As the water runs, it gradually gets warmer and warmer and warmer. This is what happens with the fat content in mom's milk - moms's milk gradually increases in fat content until the end of the feeding.
Now think about the hot water faucet again. If there is a long period of time before the faucet is used again, then you go through the "cold to hot" process once more, but if you turn the water on fairly soon after it was used then the water is either pretty warm or still hot, depending upon how long it's been since the faucet was last on.
This is how it works with mother's milk too - the longer the time between feedings, the lower the fat content at the beginning of the next feeding. If feedings are closer together, you're starting off with a higher fat content.