The status of your cashback transaction will change from "pending" to "received" once the merchant has approved your purchase. How long this takes depends upon each merchant. Some merchants will approve your purchase within a few days, whereas others may take longer.
Please keep in mind that if you've made a booking for sometime in the future, your cashback will not be approved prior to that date. This includes transport bookings and hotel reservations. For example, cashback for flight tickets will not be approved until after the date of the flight.
If you would like to earn cashback on a subscription purchase, please also consider that your cashback transaction will not be approved by the merchant until you've made at least three recurring subscription payments.
Other articles you might find interesting:
How much cashback will I receive?
What do the different statuses in my Cashback Sock mean?