Whether you're the lowest enlisted rank, or a 4-Star General (or Admiral), all military personnel get the same amount of vacation time. Military members get 30 days of paid leave per year, earned at the rate of 2.5 days per month.
Unlike civilian leave programs, however, military leave, taken over weekends and holidays count as "leave taken." For example, if you take leave, beginning on Monday for 12 days, until the following Friday after, the Saturday and Sunday in the middle is counted as leave.
According to military regulations, leave must start and end in the local area. That means, if you begin your leave on Monday, you cannot leave the local area until Monday, even if you are off-duty on Saturday and Sunday. Conversely, if you schedule your leave to end on a Friday, you must return to the local area on that Friday, even if you're not scheduled for work until the following Monday.
Normal leave is approved/disapproved by the member's immediate supervisor. Emergency leave (someone in your immediate family dies or is seriously ill), is approved by the commander or first sergeant, upon verification of the emergency (usually from the Red Cross). Emergency leave still counts against your authorized leave time (30 days per year). The commander is the approval authority if one must take more leave than they have currently earned. (Example, let's say you've been in the military for four months, and you've not taken any leave. You should have 10 days of leave "saved up" -- 2.5 days per month for four months. Assume your father gets very sick, and you need 15 days to go home and see him. You would be using five days you haven't earned yet, so the commander would have to approve it. When you returned, you would be five days "in the hole," so it would be two months before you have a "zero leave balance" and would begin earning leave again.
Except for emergency leave, and Christmas Exodus (discussed below), commanders are usually reluctant to approved leave that hasn't been earned yet. This is because, under the law, if a person is discharged (for any reason) and they have a negative leave balance, they must repay the military one day's base pay for each day they are "in the hole" as of the date of the discharge.
Leave is calculated according to the "fiscal year" (1 October to 30 September). One is allowed to "carry over" only a maximum of 60 days from fiscal year to fiscal year. (Note: Exceptions can be approved if one can show that they were denied leave due to military necessity for unusual situations, such as long-term deployments). In other words, if you have 65 days of leave on the books on 30 September, 5 of those days will be "lost" when the ca lander rolls over to 1 October. So, on 1 October, you will have 60 days "on the books."
Leave can be "sold back" at the time of reenlistment and separation/retirement. For each day of leave you have saved, you can sell it back for one day's base pay (taxable). One can only sell back a maximum of 60 days of leave during a career. It does not have to be all at one time. For example, one could sell back 10 days of leave during their first re-enlistment, then 10 days during their next re-enlistment, etc.
It's generally not a good idea to sell back leave until/if one has enough rank/time-in-service so that their base pay is high enough to make it worthwhile. For example, using current pay charts, if an E-4 (over 4 years in the military) sold back 30 days of leave, he/she would receive (before taxes) $1935.90. However, if he waited until he/she was an E-8, with 20 years of service, he/she would receive $4071.60.
If one reenlists while in a combat zone, money received for selling leave is tax free.
In addition (or instead) of selling leave back, one can take "terminal leave" when they are discharged or retire. For example, let's say you are scheduled to be discharged (or retired) on 1 September, and you have 30 days of leave "saved up." You can out process from the military 30 days early, then continue to receive full pay (including base pay, housing allowance, food allowance, and any special pays), until your official date of discharge.