Not long at all.

I ported from Verizon to an existing Sprint PCS account about thirteen
months ago. The port was processed by Verizon within four hours (which I
understand is supposed to be pretty typical) and the only reason it wasn't
complete by then was that Verizon had my most recent address on file, and
Sprint, for some reason, gave them the address where we lived when we first
moved here (the account information must exactly match, or the port will fail).

I called Sprint and corrected that problem and they sent over the porting
request again, and my number moved to Sprint by the next business day.

Just in the past week, we switched to T-Mobile so we wouldn't get hit with
tons of overages calling Sarah's mother who had just activated a new line
with TM. (My wife spends a lot of time on the cell phone calling her mom.)

We ordered two new phones and a Family Time shared-minute calling plan
through inPhonic. Credit approval took about thirty minutes, and although
inPhonic's disclaimer said the port might take 5-7 days, our phone numbers
were ringing through to our T-Mobile phones within 24 hours. This latest
port was done completely over the Web.

All of the carriers use the same clearinghouse to process ports, except AT&T
Wireless... but now that ATTWS is part of Cingular, I'm assuming that that
is no longer an issue.

Interesting theory.

I came about the same thing when my contract expired.  I wanted a forth
phone added to my account and wanted to replace my 3 existing phones.
Somehow they were trying to tell me that it was better to just get a new
4 phone plan.  It actually did sound better until I wrote EVERYTHING
down and compared.  It came down to perks from my old plan that made the
difference that the CSR's failed to point out like currently, my second
phone is free, Vision per phone is free and unlimited messaging per
phone is free.  I'd actually pay about $40 more a month if I remember
correctly with the new plan AND be on a two year contract on all four
phones.  I asked about simply adding a forth phone to my existing
account and was told that was not possible because my plan was too old.
 I went to a Sprint PCS store that same day and was able to add a forth
phone to my existing account.  I took the new phone discount which put
that ONE  phone on a two year agreement but that was it.  I actually
bought some better used phones later from a friends friend to replace 2
of my other three old phones.
I have been carrier shopping as well and every sales person from all
carriers are really good at telling you cost and minutes included for a
bare bones plan as stated on the glaring 24x36 inch posters on the walls
and doors but it gets really twisted and vague when you start talking
about the extras like messaging, internet access, roaming, pictures, etc..
Odd that they want to do business that way and you really have to keep
on your toes to prevent getting burned.