There are only two times I lubricate a chain: When it needs it and When I feel like it.

Really though, it's not so much about the time or the miles as it is about the conditions and the lubricant used. Wet sloppy or dry dusty conditions require more frequent cleaning and lubrication than relatively clean dry pavement and different lubricants (now there's a whole 'nuther can o worms) may require more or less frequent reapplication. I tend to use a dry lubricant (my current one is WD40 Dry Bike Chain Lubricant) for general road riding and dry off-road. When things get sloppy or in the winter I switch to a wet lube (Dupont Multi-purpose Synthetic with Teflon). The dry lubes I've used need more frequent application that some of the better wet lubes but that's a generalization and I'm sure there are exceptions. I clean and lube my chains when they are dirty, I have been riding in wet conditions or when they get dry and noisy (squeaky) or about once a week (about 100-150 miles for me) just because.