A general rule of how often to substitute sevenths by genre:
Blues - almost all chords have dominant sevenths
Jazz - most chords will have some sevenths, and sometimes further extensions (9, 11, 13), usually diatonic but not always, especially in jazz that has elements of the blues in it.
Classical - Mostly on the dominant chord (V7), but sometimes on the ii chord, and often on the leading tone (diminished 7th chord), and even the subdominant gets it sometimes. (Thanks to Richard, who commented below)
Metal - Never (mostly powerchords)
Rock - Depends on genre, but usually most rock is triads (with notable exceptions).
Electronic - Over most genres, sometimes minor chords will be minor 7ths to get a sort of airy feeling, but usually triads.
Pop - if there's a substitute dominant, it's a seventh almost every time. Sometimes sevenths will make an appearance, depending on how harmonically complex the song is.
Please note that these are just my opinions, and I could be wrong on some of them , but I've observed these general trends across my musical experience.