What's the longest an RFID chip can be read?
Depends on the antenna, frequency, the reader power, and the
design.
Passive RFID's have a range from a few mm's to a few meters.
In general, low frequency = near field rf = short range, higher
frequency = far field rf = longer range.
EPC HF near field tags have short ranges of less than 1
meter.
EPC UHF tags are typically 1-2 meters reliable read. Far field
range is very complex and it is possible to have multipath
reflection arrangements where 10 meters may work... sometimes.
With illegally high power transmitters, and large antennas, far
field tags can be reliably read to tens of meters or more for
non-commercial designs.
Semi-passive RFID's can be read over hundreds of meters.
Active RFID's are limited only by their transmit power and
antenna design.