When we speak of a fossil, we don't usually mean a preserved object, but rather a preserved trace of an object, like it's impression in other material. Metal objects certainly would leave impressions in the surrounding material, as well as oxides. So in that sense they can easily leave fossils.
Whether the steel artifact its self lasts for eons depends on the specifics of the metal and the chemical and thermal conditions it encounters. But under most conditions, reactive metals like steel and aluminum would react away through multiple different corrosive mechanisms listed here. There's a very nice table here of the rates of corrosion progression for a few types of steel alloy. This includes Hastalloy, which is one of the most corrosion resistant forms of steel and for that reason is commonly used in nuclear power plants. For industrial use, with "Excellent Corrosion Resistance" being less than 0.1 mm of corrosion progression per year. That rate of corrosion will eat through 100 meters of metal in a million years. Zinc-steal in contact with fresh water corrodes at a rate of at least 10 mm/year, which would rapidly destroy it over large time scales.
Another way of looking at it is that basically all iron on Earth is found in the form of iron oxide, so iron compounds do not tend to stay oxidized over large time scales. Similarly, aluminum reacts with oxygen rapidly and is hardly ever (but not never) found in nature as an elemental metal. 561
Some surface treatments such as anodization can extend this time, but will eventually fail. Some elemental metals can be found in nature, which is an indication that under some conditions they could be found as fossils. These "native metals" particularly include copper, gold, silver and platinum-group metals (see link for a full list).
If, however, a reactive metal object is kept well away from oxygen and water it may last much longer. This is the case for satellites, although most satellites orbits are not entirely stable. The longest lasting may be those boosted into graveyard orbits; standard geosynchronous satellite graveyard orbit results in an expected orbital lifetime of millions of years. Objects on Mars stand a good chance of lasting for long time scales, since they are not exposed to air or water, and are kept cold with little thermal cycling. They are also likely to become buried in dust blown by the Martian wind. The Philae comet lander may be similarly preserved, though comets tend to crash into planets after a few million years. Similarly, several of the outer planet probes may last for extreme time scales since they will be in vacuum, cold, and safe from major collisions. In particular: Voyager 1 & 2, Pioneer 10 & 11, New Horizons. But these are continuously moving away from the solar system, so while preserved, won't leave any fossils to find.
In total, the answer appears to be eons, but few objects will find suitable conditions, and the best place to look will be on Mars. 99