Unless you leave a bike outside in the rain and never care for it, it's not going to rust into oblivion because you ride in the rain on occasion.
My ol scwhinn le tour sat outside exposed to the elements for 3 months without any attention and it's not rusted to heck.. and it got rained on quite a bit in the past month or so. There is minor rust on the chain but I will take my chain lube/cleaner to it and a paper towel to clean it up.
I don't panic about the slightest bit of rust though. It's not a high dollar bike or anything. I've had it at least 3 years now, although much of that was in a garage or under a tarp. I can easily see it still being in good condition in another 7-10 years with my somewhat casual attitude toward it since I go back and clean it up if it gets a bit dirty or slightly rusted. It's gotten scratched up on the rear carrier of my motorhome due to another bike being with it. I have it in my apt at the moment so at least.