I believe the general rule and limit for all bands is either roughly 10 years of great albums OR 8 to 10 studio albums of greatness (including beginning efforts). Thus:
Genesis: up to "Wind and Wuthering" (8 albums) OR up to "Duke" (10 albums)
Yes: up to "Going for the One" (8 albums) OR up to "Drama" (10 albums)
Gentle Giant: up to "Interview" (8) OR up to "Giant for a Day" (10)
Van der Graaf Generator: doesn't really qualify because of huge hiatus in the mid-seventies; however, if the three Peter Hammill solo albums during this hiatus are counted, then up to "Godbluff" (8) OR up to "World Record" (10)
King Crimson: up to "Discipline" (8) OR up to "Three of a Perfect Pair" (10)
The Residents: counting their two formally unreleased albums ("Baby Sex" and "The Warner Bros Album"), up to "Duck Stab" (8) OR up to "Mark of the Mole" (10)
Jethro Tull: up to "Minstrel in the Gallery" (8) OR up to "Songs from the Wood" (10)
Pink Floyd: disregarding "Obscured by Clouds" or "More" (take yer pick), up to "Wish You Were Here" OR up to "The Wall"
By these definitions, I believe that bands usually have about 8 albums allotted to them - then 2 of decline - then the rest is usually pointless. Only Captain Beefheart and Frank Zappa don't fit into these outlines well - Beefheart recorded four GREAT albums, then 2 okay ones, then 3 of crap, then 1 okay one, 1 great one, and 1 okay one (in that order). Zappa, however, cranked out so many albums I simply hesitate to examine any patterns in his quality (given I haven't listened to any past "The Grand Wazoo").
Try this mehod yourself on other bands; it usually holds true - and if it doesn't, then there's usually an explaination.