It might just be a bit of software-pleasing dance, but (this is secondhand information, fwiw) the usual practice for doing "weekly" work seems to be to just agree on an hourly rate and then agree that you're going to bill them 40 hours for "one week of full-time work". So saying "I'm going to work on this full-time for the next 3 weeks" translates in invoice-speak to "I'm going to send an invoice for 120 hours of work at $agreed_rate", treating "40 hours" and "1 week" as just alternate units for the same quantity, rather than sending a detailed statement that you spent 2.5 hours on Task A and 4.5 on Task B. So I guess it's effectively weekly billing in a roundabout way. (This is mostly Lockheed-type companies that I hear about, which seem to treat single-person consultants as effectively "outside employees", and only like to negotiate custom terms with giant consulting firms like KBR.)