It is preferable if the employee or a relative can come in and take away his personal stuff if he is going to be off for a long period of time. Unfortunately due to the nature of this type of leave, it is not always possible.
There certainly should be no expectation that the desk will not be turned over to a different person (such as a temp replacement) or even that the office will still be in the same location when this person returns.
When you are out for an extended period, the company may need to use the space. We had one person who was out for six months for an illness whose possessions got moved 6 separate times in that time period as we hired many new people and tried to find the space to put them in.
The critical thing is that the supervisor take care to make sure the personal possessions are not lost.
But remember, this is not an employee's desk, it is a company desk. The company can do what it like with it. The person's computer may also get reassigned as it too is company property. After all if I hire a temp to do your job while you are out on six months of leave, I am not likely to buy a new computer for that person. This is one reason why it is almost never a good idea to store much personal stuff on your work computer.
People get attached to their space. But remember, you don't own the desk, you don't own the computer, so don't leave anything there that you would not want others to see or that it would pain you to lose. Most companies will try hard not to lose your physical stuff if they need to take the space for someone else, but if the time period is long enough, it is easy to lose stuff.