Accordingly to Walmart Reward MasterCard website, it says any transaction made on the credit card will be updated on "Online My Account" up to the minute, and customer service representative will also have the same up to the minute transaction information.
However, from my personal honest experience, realistically it takes between 1 day to 2 days for transaction to appear on your "Online My Account" for your Walmart Reward MasterCard. When you log into your Account online, you will typically see "Transaction date" and "Posted date", and you will notice difference of 1 or 2 days between transaction and posting date.
If you pay directly for your Walmart Rewards MasterCard balance at a Walmart Customer Service Desk, it will still take about 1 or 2 days for the payment to appear online at "My Account". You need not worry if you don't see the transaction online immediately, since you will be given a payment receipt by the Walmart Customer Service Desk. Hold on to this receipt just in case your payment doesn't go through for whatever reason.