According to Sesame Street Unpaved, most Muppets are replaced every five years, but Cookie Monster, Bert, Ernie, Grover, and Kermit are replaced every 10-12 years. Though it seems there are multiple puppets in use for the same characters those times. Obviously most of the season one Sesame Street puppets (as well as the season one Muppet Show puppets) were replaced or slightly altered. For season two I think they just made some changes to the faces of the original Ernie and Bert puppets (in addition to changing Ernie's shirt), and those two were completely rebuilt in season three.
Despite what Sesame Street Unpaved said, it seems like most of those characters did get rebuilt in less years than what was said. It seems Bert and Ernie are usually rebuilt at the same time, but the puppets were replaced in 1971, again in 1974 or 1975 (though I think Bert had changed by 1973... The Bert in season five's Doin' the Pigeon looks like a late-1970s Bert to me, but maybe it's because that puppet had legs), again in 1982, and then after that I can't quite keep track of the changes. It seems like Cookie Monster is the only first season puppet who didn't change for season two (though there's something about the Cookie Monster puppet used in the first episode that's different from the puppet used in other season one appearances I've seen.
While the Bert and Ernie puppets were rebuilt and replaced in 1982, it seems they still used the 1975-1982 puppets on occasion, in Big Bird in China and Follow That Bird. In The Muppet Movie, they started making Kermit out of a different type of green material (which had less noticable felt texture), and this new Kermit was used for the last two and a half seasons of The Muppet Show, but an earlier Kermit with more noticable felt texture was used in at least two season 11 segments (Kermit News: Foot Snuggies and Disco Frog). In fact it seems to me like two different Kermit puppets are used in Kermit News: Alice in Wonderland. It looks like the early 1980s Kermit when he's in his trenchcoat, but after he shrinks out of his clothes he looks like a 1976-1979 Kermit.