When using Invisalign to straighten your teeth it can take between 12 to 18 months to straighten your teeth with Invisalign.
You must wear the Invisalign as often as your dentist says too otherwise your teeth won't straighten or take longer to straighten.
Within 6 to 10 months you should see straighter teeth and after 12 months to 18 months you should have straight or at least pretty straight teeth.
For some people it might take a 2 years to see really straight teeth depending on how crooked your teeth are.
Just make sure you use the Invisalign and wear it as often as you need too so that your teeth properly straighten.