The answer to this depends on how whether or not your "force-body" detects that it has a problem.
Most of the answers have given cellular replacement rates and mentioned some cells don't get replaced, or that there would be problems with bones, etc...
However, this is all under normal homeostasis conditions. In most cases, there isn't a magic clock that tells your body to replace cells; your body replaces cells when it detects damage. The idea that the body completely replaces all of its cells on average every 7 years is mostly correct, but that's not due to a special clock; it's just on average how long cells last. The body doesn't replace parts that don't break down. Fast reproducing cells are in areas where they're likely to get damaged quickly and need constant replacement, slow reproducing cells are in areas where they don't get damaged. This is why your skin and digestive tract get replaced a lot; they're exposed to the outside world. Your brain isn't, so your neurons don't accumulate damage as quickly and thus don't need to be replaced.
But even cells that aren't replaced have to rebuild themselves and fix damaged parts. Entropy demands this. There are a lot of chemical reactions going on in a cell, and those reactions can damage the cellular machinery. Your neurons may not be replaced by new neurons, but the ones you have are constantly replacing and fixing their constituent parts (read: atoms and molecules).
All of this bodily restructuring is controlled through complex chemical signalling mechanisms. Cells detect whether or not they're damaged, and attempt repairs. If the cell is too damaged to repair itself, it decides to die, allowing itself to be replaced by a new cell. These are the processes of catabolism (breaking down) and anabolism (building up), and they occur in every part of the body without exception. Even your bones are constantly being broken down by osteoclasts and rebuilt by osteoblasts in response to signalling from the thyroid and parathyroid.
Now, the question is whether or not your cells can detect that the "force-atoms" aren't real and need to be replaced. This is actually where things can get very interesting from a story point of view.
If your force-atoms don't behave exactly like real atoms, there is a chance they won't break down at all, in which case your force-body will not be subject to the chemical stresses that drive homeostasis in the first place. Cells won't age and won't break down, and your force-person will be immortal. They also won't replace any atoms and will only rely on food/water for energy. At best you have a person full of ATP and not much else, maybe some real water.
Assuming your force-atoms are subject to the same kind of entropy and uncertainty as real atoms, we could reasonably expect that your force-person will replace itself at the same rate as a fully physical person. In this case, your force-body will very slowly replace parts of itself, but never a complete whole. This is because the parts that normally won't break down won't break down, and so will remain mostly composed of force-atoms. But, it is conceivable that on a long enough timescale, all of the atoms would be replaced. Unfortunately, this timescale is likely longer than an average human lifespan.
However, as I mentioned before, homeostasis is actively regulated. The body understands what's broken and what needs replacing, and will do whatever it must to maintain that balance. If your body detects that the force atoms aren't real, and thus "damage," it can ramp itself up and replace them rather rapidly. Cells will see their force atoms as not real and seek to replace them. Some cells will be replaced entirely, etc... Even neurons would eventually repair themselves in this case by replacing their machinery. This is essentially how wound healing works.
There is a problem with this though: a cell that has sustained irreparable damage will commit suicide through apoptosis, and a cell detecting that all of its atoms are completely fake may decide it is too damaged to repair.
In this case, your force-person dies a rather slow and painful death over several weeks in a manner that would resemble radiation poisoning. Not a fun way to go.
The key here, if you want your force person to completely replace their atoms, is to ensure that the cells detect damage that is repairable and attempt to do so, with them having a preference for real atoms and molecules over force-atoms. In this case, your force-person would replace themselves rather quickly, probably in less than a year, if they were well fed with a highly nutritious diet. Their force-bodies would also suffer from an insatiable hunger at all times as the body desperately signaled to increase nutrient intake in order to repair itself.
As an aside, there are also some deeper questions you'll want to think about. A human being isn't just all of the cells that make that human up, but rather a collection of human cells and commensal flora and fauna: a bacteriome. In fact, numbers-wise you are made up more of your bacteria than your actual human cells. These bacteria can't survive without you, nor you without them. They live in your gut and process certain foods into vitamins for you, they live on your skin and keep you protected from pathogens, they're even involved in determining your weight. They are you, and also aren't you. Is the bacteriome also made up of force-atoms, or are they real living bacteria?
