This is such a common question in EPC/UHF, and there are about 100 ways to answer it. The best answer I ever heard breaks RFID down into simple terms. RFID is the coupling of two antennae: the reader antenna and the tag antenna. There is a simple relationship between them and two primary ways to affect read-distance. The reader antenna broadcasts power, and the tag receives it. If you increase power, you increase range. The second way pertains to tag size - the bigger the tag, the longer the range.
Okay, now that we are all RFID experts, there are a number of other variables people will gladly point out. However, those laid out above are the basics and reduce the question to simple terms. So, if you have the best of both worlds, an EPC/UHF passive tag will read 25-30 feet away with good construction and equipment.
Today, read-range is not the problem very often. The main issues are controlling the RF read-zone as well as understanding where a tag might be. This is why the best problem-solvers out there have advanced software filters and an understanding on how to choose the proper antennae and reader platform for a solution.