It decreases rapidly (as 1/r2, where r is the distance from the magnet), but still may extend a considerable distance from the magnet, depending on its strength and upon the presence or absence of shielding. In reality, absence of shielding should never be encountered on a hospital site—an unshielded 0.5T magnet would produce a field strength of 0.5mT at up to thirty metres from the magnet! And fifty metres for an unshielded 1.5T magnet!
Further reading on this topic:
Books: MRI From Picture to Proton p13-14, 187