There isn't really a standard length of time - depends on supply and how much or not it might be a "paper" launch. Generally from when the CEO of the company stands up on stage waving the card around you are looking at 1-2 months before they appear in pre-builds but supply might be another matter.
. There are plenty of things they can do to make a card slower and anyway the 4080 is rumoured to be based on the bigger 103 chip vs the 104 in the 4070 (the latter may be clocked higher to meet performance targets, hence the higher power draw as you go beyond the sweetspot for efficiency).