It depends on the contract. For the most part, the scrutiny of government auditors ensure that you actually can't work more than 40 hours in a single week. Sometimes the contract also ensures that the work cannot be done outside of a government-controlled facility.
So if you're the actual do-the-work peon instead of someone in any of the several tiers of useless middlemen, you work regular 8 hour days, get 10 or 11 paid holidays, and when you get home you are simply done with work until the morning of the next workday. It does not matter much if you do your job well or do it poorly, because there are so many other people working on it that you will never get recognition for a job done well, nor will you get any blame for doing it poorly, so long as it does what it says on the box.
The deadlines also vary by contract, but as every layer of middlemen adds its own padding, it wouldn't be remarkable to have two years to develop yet-another-CRUD-app with zero scalability, compatibility, or interoperability requirements.
No one doing the actual work is a direct government employee, because the uniform pay schedules are completely incompatible with industry norms. The government employees are simply there to make sure the contractors do what is required.
The net result is that you can get all your work done in just 2 hours and look busy for 6, or amble through it at a sloth's pace, gold-plating everything, achieving 100% test coverage, using ordinary software as a teaching lab for industry best practices, and such.
There are plenty of downsides, of course. The work is never glamorous, and you never really have much say in what you do or how you do it. You have zero job security, as you could be out of work if the wind changes direction in Washington, DC. The codebase will always be complete garbage when you are first introduced to it. The unimaginative and slow environment is not stimulating, so you absolutely must have a hobby or side-project that can engage you mentally outside of work.