This is what the American Heart Association recommends, straight from the horse's mouth:
The American Heart Association (AHA) recommends limiting the amount of added sugars you consume to no more than half of your daily discretionary calories allowance. For most American women, that’s no more than 100 calories per day, or about 6 teaspoons of sugar. For men, it’s 150 calories per day, or about 9 teaspoons. The AHA recommendations focus on all added sugars, without singling out any particular types such as high-fructose corn syrup. For more detailed information and guidance on sugar intake limits, see the scientific statement in the August 2009 issue of Circulation, Journal of the American Heart Association.
So 100 calories and 150 calories are half the "daily discretionary calories" recommended for most American women and men respectively.
The linked paper gives a lot of relevant information. First of all, here's the definition for added sugar:
Added sugars are defined as sugars and syrups that are
added to foods during processing or preparation, including
sugars and syrups added at the table.
There are several reasons the article gives for wanting Americans to reduce the amount of added sugars they consume:
Cross-sectional studies in humans link soft drink consumption
with higher energy intake, greater body weight, and poor
nutrition and suggest that excessive fructose consumption
is playing a role in the epidemics of insulin resistance,
obesity, hypertension, dyslipidemia, and type 2 diabetes
mellitus in humans.
An emerging but inconclusive body of evidence suggests that
increased intake of added sugars might raise blood pressure.
Although the trends were not consistent for all
age groups, reduced intakes of calcium, vitamin A, iron, and
zinc were observed with increasing intake of added sugars,
particularly at intake levels that exceeded 25% of energy.
Over the past 30 years, total calorie intake has increased by an
average of 150 to 300 calories per day, and approximately
50% of this increase comes from liquid calories (primarily
sugar-sweetened beverages). At the same time, there
has been no apparent change in physical activity. Hence, it
is likely that weight gain over the same period must be related
in part to increased intake of added sugars, even though
research tools thus far have been insufficient to confirm a
direct link. This likely results both from the fact that obesity
is a multifactorial condition and because it is extremely
difficult to identify, much less quantify, the relative contributions
of each factor in epidemiological studies.
The article also cites this article by the World Health Organization, which makes a similar recommendation:
In 2003, the World Health Organization
stated that excessive consumption of energy-rich foods can
encourage weight gain and subsequently recommended limiting
the consumption of added sugars to 10% of total