It seems that you are referring to notification devices, not detection devices in a smoking room. According to the ADA accessibility guidlelines, a visual notification device should be mounted 80" above the highest floor level within the space or 6" below the ceiling (to avoid smoke obscuration), whichever is lower. Also, according to ADAAG, a visual device is required in a room if it is considered a common area. I would consider a smoking room a common area. Refer to NFPA 72 for proper strobe coverage.
Just curious, is this area sprinklered?