Almost exclusively late at night.
Songs usually start with a single good line or a good couplet coming to mind. If I've got that, that's like the corner of the jigsaw - the rest builds up around it.
Sometimes I'll find a nice riff or chord sequence on a guitar which I can wrap something around, but usually that just ends up as the basis for an instrumental thing rather than a song. And I've not (so far) had enough inspiration to put a complete instrumental together, just a few nice sequences of riffs so far.