Once a client initiates payment, how many does the client have to cancel the payment?

It's lovely to see you here today, Brian8,

I can add some information about canceling customer payments in QuickBooks Online.

At this time, the option to let your clients revoke their invoice payments is currently unavailable in QuickBooks Online. This is, however, a great idea to add in QuickBooks so I'd encourage you to send your product preference to our developers. Let me show you how to send a request:

1. Click the Gear icon.
2. Select Feedback.
3. Enter your feedback and suggestions.
4. Click Next.
5. Select Feedback for the Category.
6. Click Send message.

If this is a credit card payment from your client, what you can do is to void the payment the same day you process the refund. Please be reminded that 3 PM PST is the settlement cut off time and anything after that is then considered next business day. Here's how to do that:

1. Click the Sales menu from the navigation panel.
2. Go to the All Sales tab.
3. Click to open the payment transaction.
4. Click More.
5. Select Void.

That should do it. For more information about voiding a credit card payment, please check out this article: How to void a credit card payment or process a credit card refund

Let me know if there's anything I can help you. I'm looking forward to working with you again. Have a great day!