It's not very clear to me if a Hunter has to pick an Animal Focus and stick with it or if they can switch between them as often as the ability allows.
Pertinent text from the PRD
Animal Focus (Su): At 1st level, a hunter can take on the aspect of an animal as a swift action. She must select one type of animal to emulate, gaining a bonus or special ability based on the type of animal emulated and her hunter level. The hunter can use this ability for a number of minutes per day equal to her level. This duration does not need to be consecutive, but must be spent in 1-minute increments. The hunter can emulate only one animal at a time.
The hunter can also apply one of these aspects to her animal companion. Unlike with the hunter herself, there is no duration on the animal aspect applied to her animal companion. An aspect applied in this way does not count against the hunter's minutes of duration per day—it remains in effect until the hunter changes it. The companion's aspect can be the same aspect the ranger has taken on or a different one. The hunter can select or change the animal foci on both herself and her animal companion as part of the same swift action.
If the hunter's animal companion is dead, the hunter can apply her companion's animal focus to herself instead of her animal companion. This is in addition to the normal one she can choose, and (as with a companion's focus) remains in effect until the hunter changes it instead of counting against her minutes per day
From the text we can see that a hunter can choose a focus for herself and then she has that focus for one minute. You can use X times per day where X is your hunter level. Each time you use this you expend one use (duh). If you're level 2, you can use your ability take Bat and you have that for one minute. If thirty seconds into it you decide you need Bear instead, you can use your swift action for that round and grant yourself bear for one minute--cutting off your remaining time on bat and giving you one minute of bear.
Master Hunter (Ex): At 20th level, a hunter becomes a master hunter, able to track down foes with ease. She can always move at full speed while using Survival to follow tracks without penalty.
Additionally, each day when the hunter prepares spells, she chooses one animal focus to be active on herself for the entire day. This focus is in addition to using her animal focus class ability (including the additional focus ability she is able to use on herself if her animal companion is dead).
At 20th level, she can choose an aspect at the start of the day to have active all day--this is outside of the 20 uses of the standard ability and does not count against the limit nor interferes with adding a different focus to herself.