Hardwired smoke detectors with battery backup are required in some places. They are wired in a series so when one alarm sounds, all sound. Running the cable is the hardest part of the job -- the time required depends on the layout of your home. In addition, you'll need to tie in to a single source of power.
About 5 hours to run cable and install three detectors
Voltage tester, drill, 1/2-inch bit, drywall saw, fish tape, screwdriver, strippers, long-nose pliers, lineman's pliers
Installing boxes; running cable into boxes; stripping, splicing, and connecting wires to terminals
Smoke detectors, boxes, 14/2 and 14/3 cable, electrician's tape, wire nuts