There is no limit to it; why would there be any limit to it? Things do what they say they do, and nothing in the psionic body description indicates that it has any kind of cooldown—aside from the simple fact that if you don’t exist, you can’t die again, so it can’t be used again until you return and then would die again.
Even dying, for real by rolling less than 10, doesn’t really stop the ability from working—if resurrected, you would still have this ability and so be able to continue using it.
And again, the question remains: why should this be limited? A 20th-level adventurer of any stripe has a variety of methods of overcoming death. Resurrection has been available to clerics of that level for seven levels—and raise dead, which is nearly as good, for literally more than half their career. Sure, raise dead or resurrection has some drawbacks (the caster cannot cast anything else that day, the subject takes a penalty that takes a few days to wear off), but psionic body has a drawback too—you can’t do anything for 1d3 days. That’s better than raise dead or resurrection, but class features are supposed to make you more powerful—particularly 20th-level ones!