I'm from the old school. I grew up in the 80s before the internet. In fact, I can remember what it was like before rappers had albums. At first artists would tape their shows and sell them like albums. They'd release a 12" single once or twice a year and tour off that. In the early/mid 80s rap albums would come out one at a time. Like Grandmaster Flash & The Furious 5 might release an album. Then a few months later Kurtis Blow might release an album. Then a few months later Afrika Bambaattaa might release an album. Then a few months later The Sugarhill Gang, Then Run DMC, etc.
Then in the late 80s music videos and rap albums became more prevalent.
By the mid 90s rap albums were common. By 1994 it was completely normal for a group/MC to release a single with a video and the album would drop the next month.
Then sometime in the early 00s, I guess it was because of CDs and how easy it was to duplicate a CD and maintain CD quality, mixtapes became more common. An artist might have a mixtape with unreleased music to accompany their album.
But to answer the O/P When they first drop they should release a major label release, then 12 months later they should release another major label release. It doesn't hurt to drop mixtapes and freestyles in between albums. After they've made their mark they can afford to wait 2 or 3 years between albums.
As I understand it, the artist doesn't quite understand the industry when he first comes out, it takes about 5 years to figure things out. In other words, he might be hot when he first comes out, but he's not making any money. By the time he figures out how to make money he doesn't have to release music as often.