I can pretty much guarantee you that my cows would NEVER wean their calves no matter how long I kept them on the dam. My cows' milk production does not decrease very much.
As mentioned, a lot of farms have a "set date" that they wean. Generally, calves are weaned between at an AVERAGE age of 6-8 months. Since most herds have at least a 60 day calving/breeding season, there is a 2 month spread of age. All beef breeds have an adjusted weaning weight to a 205 day age. By all breeds using this standard, you can compare weaning weights. You can't really compare the actual scale weight of a 6 month old calf to a 8 month old calf, so there is a formula to adjust the weights to a 205 day figure.
Say, your 6 month old weighed 575# and your 8 month old weighed 750# - which cow produced the best calf?
575# / 180 days = 3.19# X 205 = 655#
750# / 240 days = 3.125# x 205 = 641#
In an actual PB operation, the BW is subtracted from the scale WW, then divided by the number of days old, times 205, PLUS the BW back in.
But, if you are culling the cow, or your cows are severely stressed due to a drought, you can wean as early as 4 months of age. Providing a "creep feed" for a few weeks prior to weaning, helps lessen the stress. Weaning is a management balancing thing - you do it when it best suits your cows needs & your farm operation.