How many days do you have to pay for your repossessed car before they sell it?

None. The lienholder may dispose of the vehicle as soon as they physically receive it (and probably will do so as soon as they can; they're not in the car storage business). The time to worry about paying for it was before it was repossessed.

This might be different if your finance contract says otherwise, or if there's a specific law in your state requiring the lender to hold the vehicle for a certain time.

That said, you might get lucky. Call them as soon as you discover it's been repossessed and see if you can work something out. If they believe you have a reasonable chance of paying them in full in a short time, they're more likely to wait and see.