What is proper thank-you note etiquette from house guests after
they've departed and returned to their own homes? Should one expect a
thank-you note via snail mail, or are profuse verbal thanks previously
rendered in the driveway sufficient? Does the nature of the house
guests make any difference (i.e., close family members vs. friends)?
Does the duration of their visit make any difference?

Greetings Nautico,

Some suggestions I located:

"When you?re done visiting ? and this applies to any visit, regardless
of Internet access ? be sure to thank your hosts for their
hospitality. At minimum, do this verbally as you leave. "Thank you for
letting me stay, I had a great time." Among couch-surfing geeks, this
is the lowest common denominator of civility: any less is outright
rude. If you?re staying in a family home and have your own guest room,
or if your hosts have cooked meals for you, a little more is probably
in order: a small gift such as chocolates or a bottle of something
nice, or an old-fashioned thank-you note (especially if you?re staying
with someone?s parents). Handwrite and snail mail your thank-you note,
as emailed ones only get half points, if that. And on all occasions,
if you are able to, it is a good idea to offer hospitality in return.
"If you?re ever in Ottawa, let me know. I?d love to have you come and
stay at our place."
http://geeketiquette.infotrope.net/archives/2005/03/02/houseguests-and-home-networks

How to Write a Thank-You Note After Being a Houseguest 
http://www.ehow.com/how_9735_write-thank-you.html
[Some great tips here - comprehensive and concise]

"As a houseguest, always write a thank you note to your hostess within
a few days after the visit."
http://www.kappakappagamma.org/TemplateRedirect.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=1581

"Employ these simple tips to being an outstanding houseguest, and
ensure that both you and your hosts will enjoy your visit:"
http://www.advancedetiquette.com/newsletter/aug_issue.htm
[More great tips here]


I didn't cut and paste all the examples because a few were the entire
article and I strive to respect copyrights.  Duration didn't seem to
come into play nor did family versus non-family guest.  All seem to
agree that snail mail is the proper way to thank a host/hostess and if
you didn't bring a gift for the host/hostess when you arrived, I
suggest sending something you know they'll enjoy as well as offering
your home for their comfort if they travel to see you.


Should you have any problems with any of the links loading, please
request a clarification before rating my answer as I am happy to
respond further.

Best regards,
journalist-ga


SEARCH STRATEGY:

etiquette houseguest thank you note
proper etiquette houseguest thank you note

Outstanding answer.

Dear Nautico,

I wanted to thank you for the lovely time I had while answering your
question.  Your query was a pleasure to research, and your kind words
and generosity are a wonderful surprise!  I'm delighted you are
pleased, and that I was able to be of assistance.

Best regards,
journalist-ga,
in her best Emily Post  ;)

Yes, indeed, the less common it becomes to write a thank you note, the
more it is appreciated.  Emily Post and co. probably say that it
should be sent within 24 hours, and this is really easier:  no need to
explain why it is later and what you may have been doing;  the
impression of the visit is fresh and comments about what was so
enjoyable come easier.  And one has a wonderful sense of having done
the right thing!
As a house guest, it is very nice to bring something along when you
arrive, flowers or chocolates, or a better bottle of wine, but that
only obviates sending something later, does not obviate the thank you
letter  - which can be decisive, if you want to be invited again,  or
decisive for impression if you have to be  - the in-laws, for example.
 :-)

Do I?  I try to...