How soon would i find relief from zoloft (sertraline) for panic disorder w/agoraphobia? would i need to be at the therapeutic dose for it to work?

A bit of time: Around 6 weeks. Therapeutic dose is an estimate based off population standards, so if you're a dwarf or a giant it may vary somewhat.

Zoloft (sertraline) & Panic D/O: Zoloft requires at least 4-6, and probably at least 8, weeks to show its effects. I see you're on xanax (alprazolam) too -- this can be used temporarily while you are getting accustomed to the zoloft. You may find that zoloft prevents panic attacks sooner than this, but 6-8 weeks @ therapeutic dose for a good trial.