Considering zoom teeth whitening... need advice!!!

Hey! I have yellow teeth and I want to whiten them. It's my elder sister's marriage on July 1st and I'm super excited. I have made many plans for the function. Because of my smoking habit, there are stains on my teeth and my mom is compelling me to get zoom teeth whitening procedure from a clinic in Burlington. I just want to know few things before getting it done. How long will it take to show the best result? Also, need tips to maintain them? Is there any side effect for zoom teeth whitening?
 
Our friends had their teeth whitened at the sonoran spa and it whitend them 7 shades in 1 setting. They were happy and said price was very reasonable.
 
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