If you notice changes in ride (softer/harder) or traction (more/less) over the same roads that cannot be attributed to temperature or precipitation changes, it's time.
I check them more or less depending on changes. Since the pressure doesn't really change, it's basically whenever I feel like it. Mom's old car (rarely driven) looses about five to eight pounds a week; less if it's actually driven.
If you check it in a week and nothing has changed, check it in another week. If it hasn't changed over two weeks then check it in a month. If it hasn't changed in a month then check it at oil change intervals.