Hey, so I feel like I'm in a songwriting rut where any riffs I come up with are 1-2 bars long that just end up repeating themselves, sometimes with small variations and changing notes but overall the same thing. However, when I look at tabs and listen to bands like Periphery, the riffs seem to be a lot longer (example: Insomnia after the initial chugging, even the chugging has a more varied rhythm than I could think of) and more interesting than what I can think of. How can I think in terms of longer riffs?
OK, well lots of band write repetitive riffs. It's a boring example, but Enter Sandman is three repeats of the same figure followed by a 2nd figure. So it's a 4-bar riff that uses 2 ideas. According to Kirk in the Some Kind Of Monster film he wrote it as a 2-bar riff and that's how it stayed, until James suggested he repeat the 1st figure 3 times and use the 2nd figure as the 4th bar. So there's one idea ...

Repeat the first figure (the first bar) of your riff a number of times and use the second figure (the 2nd bar) of your riff as a 'tail' to the first bar's 'head'.

As far as Periphery goes I'd never heard of them, but I've had a quick listen to Insomnia ...

The riff starting at 0.31 is composed of 4 cells - the first cells sounds like 7 notes of a descending arpeggio that's been buggered around with a bit. The next cell sounds like the same arpeggio in reverse - it's also 7 notes long. The rhythms of these two are identical - straight 16ths. There's a couple of chugs. Cell 3 is another ascending thing, same rhythm but continues for 11 notes. Cell 4 continues the ascending thing further up the fretboard and extends for 15 notes.

So this is still fairly repetitive (arpeggiated figures), but the idea's spun out in a different way - descending/ascending/ascending a little further/ascending further still.

So there's another thing you can do.

Write a 1 bar riff, reverse it, extend it, extend it further.

It's not going to be quite as simple as that makes it sound - it's unlikely you'll like the reversed version of the riff so you might need to muck around with a bit to make it work - but that's the general idea.
Quote by Hail
oh shut up with that /mu/ bullshit. fidget house shouldn't even be a genre, why in the world would it deserve its own subgenres you twat
I struggle with this sometimes too. One thing that helped to inspire me was something I noticed about a few Lamb of God songs back when their guitarists were doing a column in Guitar World(like 8 years ago, damn where's the time gone!)

Anyway, I don't remember which songs exactly, but a few of their riffs are fairly repetitive but they would be structured so that the bars are constantly changing. The bars would go like this:

A | B | A | C | A | B | A | D

So basically every other bar is a repeat but the second bar is different or it could be thought of as a 4 bar repeat with 2 endings, but because bars 1 & 3 are the same it makes it feel like its repeating more often with different endings.
I'm an

I'm Good at Math
Something from Messiaen ... additive rhythms.

Take a phrase in constant 8th notes against a constant back-beat. Now play exactly the same notes, but alter the duration of one (or more) of the 8th notes so it's a dotted 8th note. This will shift the rhythmic emphasis of the notes against the back-beat.

You can also throw your rhythm off-kilter by inserting a couple of chord chugs (like in Insomnia), a short phrase, or just some silence between segments of the riff and then continuing where you left off. If you've got a good drummer in your band they might be able to keep the back-beat going in one hand/foot, while emphasising the off-kilter rhythm in another hand/foot as you play (or you can always call on Doktor Avalanche to do it for you if you've no drummer).
Quote by Hail
oh shut up with that /mu/ bullshit. fidget house shouldn't even be a genre, why in the world would it deserve its own subgenres you twat