The answer is that that’s the wrong question!
Beeminder doesn’t care too much about when you do the work; it just forces you to stay on the good side of the yellow brick road every day. So, for example, if you start the goal Monday, you can do all your work Monday and coast the rest of the week, but you can’t procrastinate and make up a week’s worth of work on Friday. Because you’ll have fallen below the average pace before then. You don’t have to work a little every day, if you’ve already done enough work to stay on track for awhile.
When you create a new goal, you can opt to start with some extra days of safety buffer to lengthen the time between create-date and must-start-working-date (which is 2 days by default). So to 100% safely start, say, a monthly goal, you’d want to use 30 days of safety buffer. But as the goal goes on, you’ll always be forced to maintain your goal rate over the last 30 days, no matter which 30 days those happen to be.
(We get this question a bunch, which means we need to do a better job explaining and/or making it clear up front! There’s also a semi-old blog post describing these types of “chunky” goals.)