I think it depends on the school.  Some schools are very particular that you are getting certain classes from their faculty.  If you are coming from a state school and going to an ivy/private school, a lot of times they will make you take all 3 years, but you can get out of some of the electives to free up your schedule.  A lot of schools like to keep you there for three years though because they like the tuition dollars.  

At some schools it happens fairly frequently - i.e. as much as 20% of the incoming class may get advanced standing.  At other schools it happens pretty much never. 

When it does happen, it almost always requires that the undergrad program you attended had at least 6 semesters of studio.  Work experience isn't usually considered, first because the accreditation rules don't take that into account, so if a school is offering advanced standing they have to be able to prove that they're only offering it to people who can demonstrate equivalent academic work to what's being waived.  And second, most architecture schools don't exactly look highly on professional work - they don't care what tasks you worked on as a team member on a real-world project - they care much more about the series of squiggles you sketched as part of your iterative design process on your studio project: home for a nomadic blind graffiti artist.

If this is a big issue for you in choosing grad programs, make sure you understand what "advanced standing" gets you at each program.  At some it can shorten a 3-year program to a 2-year program with no big strings attached, but at others it only gets you exempted from 1 semester of studio so if you're hell bent on finishing in 2 years it requires at least one summer studio (which many schools don't even offer).

In some cases even if you don't get advanced standing you can get waived out of some or most first-year core courses except studios, if you took those courses in an undergrad program and still have the transcripts and got at least Bs in them - but then you're usually just being waived from the specific courses, not the required credit total, so it won't usually translate to finishing early.

Great post

^Plus, the schools charge considerably more for their 'graduate level' courses compared to their undergraduate courses because.....they can. The student, or eventually the taxpayer in a large number of cases, can pick up the bloated tab.