After an academic paper has been presented at a conference, how long does it usually take until it's available for people to read online?

It think it’s possible if not likely that some of them will never appear online. Conference papers, some of them are presented and aren’t yet or may never be published, at least not in the same form that they are presented at conferences. You may have to contact the author (s) if you want to read a copy. Anyone who knows more about this, please correct me if I am wrong. And, because that is, I think, the case with these things, there is no guarantee that a paper presented will ever be available online let alone on some standard, specific time schedule.

@lilycoyote is correct. The paper doesn’t appear online unless 1) the conference requests it from the presenter and 2) the presenter/author chooses to allow the conference to publish the paper online. In many cases, if the presenter is employed by a university, the paper they write is owned by the university they work for, and not the author. Often the papers are published in academic journals but it can take 6 months to a year for the paper to appear.

Sometimes they are available from the university library. But your best bet would be to contact the author and ask. In the example you cited, Dr. Boulton’s e-mail address at Notre Dame is readily available.

University library would be your best chance. I don’t have experience with history papers, mostly scientific and medial and sometimes I have to pay a subscription to an online publisher if I want more than just the abstract.

I found the program for the conference:
http://www.acmrs.org/conferences/MAA_2011/MAA11ProgramFinal.pdf

Here’s the professor’s email address and details: http://nd.edu/~medinst/faculty/bios/boultond.html

If you can’t get it from the University, the paper may be picked up and published in a number of journals, like his three previous papers so you could write to them and see if they’ll be publishing it…

Recent Publications
Alta Studia Heraldica: The Scholarly Journal of the Royal Heraldry Society of Canada, vol. 2 (2009), and vol. 3 (2010).
The Ideology of Burgundy: The Promotion of National Consciousness 1364–1565 (edited with Jan Veenstra, Brill, 2006)
The Knights in the Crown: The Monarchical Orders of Knighthood in Late Medieval Europe, 1326–1520, second edition, revised and expanded (Boydell and Brewer, 2000)

Probably the easiast way to get hold of it is to contact the authors directly and ask for a copy.