Some of the most popular synthetic marijuana products that are currently being smoked are K2, Spice and Mr Nice. These products, along with a number of the other legal highs that are currently available on the open markets, will tend to stay within your bloodstream for approximately 72 hours after you have used them. However, as with any type of drug this can obviously vary for a number of different reasons.
Your size and body mass will have a lot to do with the length of time the synthetic marijuana stays within your system. The larger you are, the less time it will generally stay in your system for. Obviously, if you have only consumed a small amount of the drug then it will probably leave your system more quickly, whereas if you have consumed a lot of the substance then it may take longer than 72 hours.
Your age and your metabolism will also affect the rate at which the substance leaves your system. One important thing to remember though is that the drug is currently legal and does not show up on normal drug tests for illegal marijuana use.