Select
    Customer_ID
    ,row_number() over (partition by Customer_ID order by date) r_num
    ,date
   ,total_paid
Into  #Customer_Transactions
From  Customer_Transactions
Select
    t1.Customer_ID
   ,datediff(dd,t1.date ,t2.date)  --this will calculate the difference in days
From   #Customer_Transactions t1
Join   #Customer_Transactions t2
ON   t1.Customer_ID = t2.Customer_ID
And    t1.r_num = t2.r_num+1