It’s up to you and the hospital! You’ll agree on the timing and duration of your externship with the hospital after you apply. Hospitals can recommend a specific duration, but ultimately, it will have to fit your schedule.
Typically, veterinary externships run one to two weeks, but if you're soaking up the experience (or just can't resist more puppy snuggles), they can last up to six weeks if that timing works for the hospital as well.