Google announced today that it is introducing a monthly charging limit for AdWords campaigns, which is automatically calculated based on advertisers’ daily budgets.
"Over the years, many of you have told us that you’d like to be charged a consistent amount each calendar month," Google’s Dan Friedman says on the Inside AdWords blog.
"When your daily budget is unchanged throughout an entire calendar month, we’ll set a monthly charging limit for that campaign by multiplying your daily budget by 30.4, which is roughly the average number of days in a month 
(365 days in a year / 12 months = 30.417 days/month)," he explains. "For example, if your daily budget is US$10 throughout a month, you won’t be charged more than $304 during that month (US$10 daily budget * 30.4 average days per month). As usual, actual daily spend may vary."
Google says it will transition to monthly charging limits over the next several weeks. When an advertiser edits their budget in the campaign settings, they will get a description of the change.
Google has an FAQ on the monthly charging limit here.