It depends. The IRS needs to see that the assistance you received to purchase insurance is reconciled. Form 8962 is used to calculate the amount of Premium Tax Credit based on your income throughout the year. If you made more money than what was used to provide you with assistance, then you likely received too much and will have to pay some back (unless other credits can reduce this amount). If, on the other hand, you made less money, it is possible that you may receive additional credit on your return. Either way, the IRS needs to reconcile this before they send you your refund.
Here is an answer I gave on how you may do this: https://ttlc.intuit.com/replies/6044131
In addition, here's an FAQ with more information on the Form you received from the Marketplace, 1095-A, and what you will need to do with this Form: https://ttlc.intuit.com/replies/4268797
On your second question, it is difficult to answer how long the IRS will take once they receive the form. I had one client I assisted with this in the past, and I don't remember it taking 6 to 8 weeks to finish processing, but, once in the IRS' hands, only they can truly determine how quick the process will be.
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