From the FAA AIM Chapter 2. Aeronautical Lighting and Other Airport Visual Aids
b. Precision Approach Path Indicator (PAPI). The precision approach path indicator (PAPI) uses light units similar to the VASI but are installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 SM from the runway threshold.
EDIT: from different source... The VASI only provided guidance down to heights of 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft).
So that looks like it covers both parts of the question, as others stated, down to 50 ft and out to 4nm...