Are they going to be testing specifically for it? Whether they have no reason to test for it, they won't. Gcms is Expensive, so when that urine goes into that machine, they have a list of exactly what they are looking for. Whether it is a fairly standard what's in your pee kind of tests, it will likely be for marijuana, cocaine, Pcp, benzos, opiates, barbiturates, amphetamines and Mdma-class drugs. None of which carisoprodol is. I think I saw another question where you asked about Trazadone or something like that as well. They would not test for that under almost any circumstance because it is an anti-depressant, and not considered a drug of abuse nor a controlled substance. While Somas are a drug of some low-scale abuse, they are pretty much under the radar. They are not even a controlled substance in every state.
So unless those testing you have a specific reason for looking for these drugs, they won't test for them.
EDIT: Ohh yeah, and if for some reason they do have a reason to test for those drugs (maybe they know that those are your 2 drugs of selection or something), I would assume somas probably stay in your system for around 2 - 5 days, and Trazadone would be a bit longer, possibly 1 - 2 weeks. Of course these are just my estimates, since these are not typically drugs tested for so there is little to no literature available on the time ranges.
about 3 or 4 day