I bathe Tango every 2-4 weeks, depending on how dirty she is to me. She's a retriever and still has oils in her coat, no matter how may times I bathe her she can go outside, go for a swim and be dry within minutes. Maybe it's because I use a mild, high quality shampoo but I really don't believe that dogs lose their oils if they're bathed often. Maybe it's just her though, I don't know. I can't stand dirty dogs though so I'd never let her go any more than 4 weeks.
Winston and Morgan get bathed every 2-3 weeks. Morgan is always getting into something and gets filthy quickly and Winston's coat just feels disgusting after a couple weeks. I don't know why, it's not as if he rolls in the dirt or anything, he just gets gross. Tia, the mini Dachshund get's bathed every month or two usually. She doesn't get dirty often, never smells, and always looks shiney and feels clean.