Most routers provide Wi-Fi coverage for about 100 feet in all directions, according to How Stuff Works. Range extenders or repeaters can be used to expand this distance.
The signal strength will always be highest in the room where the router is located. The signal gets weaker the more doors, walls or even appliances it has to pass through. Wi-Fi that has to compete with many other Wi-Fi signals will also be weaker in signal strength. This occurs in locations like apartment buildings.
It is worth noting that the signal from a wireless router must compete with other household electronics that also operate on the same radio frequencies. These devices include Bluetooth headsets, cordless phones and microwave ovens.