Steel frames, particularly the older or less expensive ones with relatively thick tubing walls will last a long time even if subject to a fair bit of rain and wet conditions. It is possible to rust out a steel bike frame but it takes a lot of time and concerted effort and neglect.
That said, there are inexpensive ways to protect them even from harsh conditions. A commercial product known as "Frame Saver" is available in many bike shops. It's an aerosol can containing a rust inhibitor treated wax in a solvent. You spray into the interior of the frame's tubes and stays and it dries to a waxy film that protects against almost all corrosion. You really have to strip the frame to do the job properly but it's a one-time treatment. I've used it on all of my steel bikes and I have a 1983 Trek 400 that was my rain bike for five years and the frame has no rust at all.
Others coat the frame tube's interior with linseed oil or even motor oil for rust protection but Frame Saver is the easiest to use and the cost is quite low. Amsoil HDMP is exactly the same product as Frame Saver and it available in many auto parts shops since it is sold as a motorcycle chain lube and car body undercoating. The benefit of HDMP is you get a bigger can for less money.