With The Avengers coming out today in the US, artist Jer Thorp had a look at character appearances, of which there have been 127 since 1963.
We can see a big cluster of major Avengers appearing in the first few episodes, with some other big names coming in the next few years (Vision, the Avenger with the 3rd most appearances in issues, doesn't come along until #57). While there are a couple of major additions along the way (She-Hulk & Photon in 1982), we can see that the cast of characters for the team is defined pretty early.
See Thorp's post for additional categorizations such as gender balance and robot characters. Best enjoyed in high resolution.