Clean and lube chain when it starts making a funny sound.
Change brake pads when I loose brake-feel.
Change cassette and chain once a year (coz I dont clean it enough I suppose).
Check spoke tension when I hear funny pinging noises.
Clean bike reasonably thoroughly once a year while new chain is put on.
Tire pressure between once per week (if I want pressure to go down for a comfortable ride) to few times a week (to keep pressure high so I can feel the 'handling').