A lot of (ostensibly-"non-H") yaoi has only has one or two scenes of fucking (not to put too fine a point on it) often at most, say, one or two brief scenes during the plot, and maybe a longer bit in the epilouge. Basically, if the manga description isn't all about sex, a lot of the time (about half the time, maybe?), the plot is actually what it says it is, just with the caveat that it's definite there's gay sex at some point.
Some on the other hand are complete smut-fests; author (and audience) thirst may vary.
As a special warning for the genre, however—often when yaoi-sex does appear in an otherwise innocuous-seeming story, it appears quickly and without warning (and oft-as-not with questionable-consent involved). It's a pretty hazardous genre as a whole, full of genuine WTF moments.