My design uses a round steel bar inserted through a hole in an aluminium component with a tight fit (no movement). The operating environment is dry, 20-40°C.
Do I need to worry about galvanic corrosion, and if so how long will this last before starting to corrode? Also, what storage conditions should I avoid that would accelerate the corrosion?
Very probably, no.
Galvanic corrosion required the existence of a reactive and conductive medium, which you don't have.
If there is some (for example, at least moist air, or similar) best you can do to paint both of the metals, if you can.
Generally, if none of the materials would corrode in an environment, galvanic corrosion is a non-issue (in that environment).
Aluminum and steel are very near to each other in the Galvanic Series for seawater. So even if your apparatus is exposed to a corrosive environment along those lines, it should not corrode very quickly.
The aluminum will corrode first, if corrosion occurs. If the exposed surface area of the steel is small compared to that of the aluminum, the corrosion that occurs will be very slow. This is part of the principle behind galvanized steel pipe: even if the zinc is scratched through, the remainder of it will corrode slowly and galvanically protect the steel. Because the surface area of the exposed steel is so much less than that of the zinc, the corrosion of the zinc is negligible.
Wikipedia defines galvanic corrosion as:
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte. This same galvanic reaction is exploited in primary batteries to generate an electrical voltage."
I don't see any electrolytes in your set-up to generate the required electric voltage. As long as you keep your system free of liquids that could carry such electrolytes, you're fine.
