Pretty much forever.
Most decomposition of (human) bodies is done by organisms in the environment in which the body dies. It can also be slowed or eliminated by storing the body in a dry and cold environment, which space certainly fits the description of.
One prime example of an extremely well-preserved body is the body of civil rights leader Medgar Evers who was assassinated in 1963. When his body was exhumed for a civil rights investigation in the 90's, they found that the body was almost perfectly preserved, which helped finally get a conviction against his assassins. This is likely attributed to the fact his coffin was lined with lead, which prevented bacteria and decomposing organisms from getting in. Obviously storing a body in space is even better, considering the lack of decomposers present in a vacuum.
Another natural phenomenon that results in well-preserved bodies is bog bodies. Due to the acidic water, lack of oxygen and low temperature preventing decomposition. Some bodies dating back as early as 8000 BCE have been found mummified in bogs. The 4th Century Tollund Man found in 1950 was originally thought to be a recent murder victim due to how well they were preserved.
I'm not sure how irradiating the body would affect its decomposition though. Although it would certainly kill bacteria, I would think it could also damage the body. You might be better off just cleaning the body before burial and sterilizing its uniform and coffin separately.
I would also store the body in near vacuum as well as opposed to filling it with an inert gas as I would think your main problem would be it being hit by space junk. Keeping it in near vacuum would mean that if the coffin is punctured, you won't have explosive decompression that could damage the body. It would also minimize pressure fluctuations from temperature changes. Obviously, you would want to lower the pressure gradually instead of all at once in order to prevent damage to the body. Storage in a vacuum would also help insulate the body, as radiation through a vacuum is an extremely slow way of transferring heat.
You may also want to insulate the coffin with a vacuum layer between an inner and outer shell. If you leave a coffin in orbit around a planet, you should be able to calculate roughly how large this layer would have to be to prevent the body from heating by calculating how long it would be in direct sunlight vs how long it would be on the dark side of the planet. Alternatively, you could place it far away from a star.
Your main problem is probably going to be space junk and other unpredictable cosmic events such as solar flares and gamma-ray bursts.