Ten seconds and after that I start thinking "this is not working" and I worry in about my credit card being or not being processed or being processed twice if I have to do this again.
It is good to change the message after 10-15 seconds to "it is still loading, no need to worry" or any other positively reassuring message.
Just an example is sending an email on gmail without letting the attachment to preupload. The message changes from "sending" to "it's still working".
Please note there needs to be some kind off response from the server to confirm that the process is still running or report an error to a user.