I work for the government too, not federal. The lower grades work more hours since they are hourly and are required to work 40 hours. The higher grades all work fewer hours since they are salary and have no minimum working hours. Having talked to my coworkers who have been in government work all their life almost no higher level employee works over 35 hours a week, and none will work over 40.
I have seen people who come in for 10 minutes leave and claim zero time for the day. I have seen people log into their email from home and will claim zero time for the day. Because our MOU says this can be done for salaried workers. Though only hand full will take it to these extreme level of abuse. Most I see work around 35 hours a week on average and will attempt to work at least 4 hours a day.
Here we are entitled to 70 minutes worth of breaks a day, a 30 minute one that we can use any time and a 5 min break each hour, to be used during the hour too. 90% of employees here take full advantage of this, and so do I. I take a 5 minute break almost every hour, plus a 30 min coffee break each day.