NRS 597.850 instructs shopkeepers to detain suspects on the premises. The statute defines premises as: Any establishment or part thereof wherein merchandise is displayed, held or offered for sale. Presumably this includes a store's showroom, storage rooms, and back offices. Therefore, merchants may not take suspects offsite to detain them. The statute also instructs shopkeepers to detain suspects for a reasonable length of time. This is how long it takes to recover the stolen items and/or wait for the police to arrive. So the exact length depends on the circumstances.