I have seen in many siddurim, mainly in korbonos, the phrase "if you have time, add this parsha".
How do you know if you have time? It seems obvious that if adding it would make you miss zman tefila, you don't have time.
If it's already past zman tefila and you're relying on davening before chatzos, do you have time? If it's well before zman tefila but adding this would make you late to help your wife at home, do you have time?