Back screen cracked

Apple does NOT replace the back screen on iPhone. It is fused to the phone and is unrepairable. Instead, Apple will offer you an Out of Warranty replacement phone, per the prices (US prices) in this link --> iPhone Repair - Official Apple Support

This can be handled quite expeditiously in an Apple Store as long as they have a replacement device of the same model, color and storage capacity in stock at the time of your visit.

Hey AjayMohkar,

Thanks for posting your question in Apple Support Communities.

The time that is required for your device to be repaired depends entirely on the place you go to get your device serviced, their workload, their parts available, and their time. It would be best to consult with your local authorized service provider where they can give you an accurate turnaround time and cost breakdown.

You can find a service provider nearest to you by following this link: Find Locations.

Best Wishes.