The National Center for Education Statistics in the United States surveys faculty of post-secondary institutions (see the National Study of Postsecondary Faculty page for information on methodology). The most recently available data is from their 2004 survey, with 26,110 respondents across the United States.
According to this survey, tenured faculty worked 53.3 hours a week on average, and tenure-track untenured faculty worked 53.7 hours a week:
The NCES allows you to create custom tables from this dataset using the PowerStats tool on their website. (You have to create an account to use the tool.) This is a valuable tool if you're interested in exploring these and other statistics.
For example, here's the same data broken down by rank, with the percentiles for average hours worked per week: