It is very common and probably convenient to store bikes by hanging them from the front wheel. A lot of shops do this. But if you hang a mountain bike that way for a long time, then the oil in the shocks can leak out. If you find a spot of oil all over your stem or seatstays then be sure to get your shocks overhauled before you use them again. The same thing is true for hydraulic brakes – hang your bike upside down and you will probably find brake fluid all over the floor – never a good thing!
There is no hard-and-fast rule as to when you should change your fork oil, except for this: it’s worth the effort to change the oil in your fork after its first 20 hours of use. In the initial break-in period for your fork, all the produc- tion remnants and shrapnel (small burrs from machining, casting flash, etc.) will find a new home floating in your oil.
And that can restrict your oil’s flow through valves. Flush out all the original oil and crud and replace it with new oil. After that, oil changes can happen a lot less frequently, maybe once a year to every 18 months, depending on use.