How long should we give sertraline for it to work?

Hi, Mancmiss!
Unfortunately, it does take a bit of time for this type of medication to become fully effective and there may be some side effects during the initial few weeks. Please pardon the copy/paste:
"Sleep, energy, or appetite may show some improvement within the first 1-2 weeks. Improvement in these physical symptoms can be an important early signal that the medication is working. Depressed mood, anxiety, and lack of interest in activities may need up to 6-8 weeks to fully improve."
"It usually takes 6 to 8 weeks for sertraline to work. Side effects such as feeling sick, headaches, emotional lability, depression, and trouble sleeping are common. They're usually mild and go away after a couple of weeks."
"Side effects have been reported to be generally mild and transient. They are most common during the first 2 weeks of treatment and decrease in intensity and frequency with continued treatment. They generally do not lead to treatment cessation."
That all being said you should always report any side effects to your health care provider for professional advice, especially in the early weeks of treatment.
Best regards and I hope your daughter's feeling much better quickly.