I smoked marijuana once. How long would it stay in my system?

I could give you a flippant answer but I won't (some politicians never inhaled or exhaled (imo) as an example). The truth of it is that THC is highly fat soluble and can be detected for months in blood (specifically as it is a common test) and even hair testing after use. My last blood test indicated the presence of THC 5 months after use. Depending on the person, their individual results will vary greatly. Saliva and urine testing will be far less long term time accurate and as a result clear people earlier.
If you are concerned regarding employment or pre-employment testing/screening your best option would be to purchase a standard 12 or 15 panel urine test kit and find out for yourself. It may well save you some pain.

Good luck.