Credit card payments are due at the time you submit the online application.
There is not a specific time limit, but applicants who choose to pay the application fee via check or money order will not have their applications processed or considered for admission until the application fee has been received.
UC Davis offers application fee waivers only to applicants who have participated in or are affiliated with some particular programs. See this link for further information:
At the above link, scroll down to find the information about waivers under the application fee information.
The application fee is firmly required and can not be waived or deferred for international students. Applications will not be processed or reviewed for admission until after the application fee has been paid.