Food safety standards state four hours at room temp (they mean 72, adjust if it is hotter), then discard. You would want to follow the guideline so that you don't set yourself up for liability.
Pastry creams, like custards, or any egg and dairy based product will break down. I do agree with the above poster to a point. I have one that seems to defy age too, but only one. Since we don't know the recipe the OP will be using, my posts always tend to be based on the average, or standard result.