Your question is quite vague, but in regard to your specific reference to "at any time", the answer is a resounding NO. Cancellation of any rental is always subject to the specific terms of the agreement associated with a particular rental.
Cancellation of a purchase of a timeshare from a developer is allowed only during a very specific period identified by applicable state law. In the U.S., that time period can be as short as 3 days or as long as 10 days, depending upon the individual state in which the contract was executed. In most states, the rescission (cancellation) period is 5-7 days after contract execution.
With the noteworthy exception of the state of Florida, which allows a 10 calendar days cancellation period for either developer direct or resale timeshare purchases, I am not aware of rescission (cancellation) rights existing at all in any other states for resale (i.e., not developer-direct) timeshare sales.
Hopefully, some of the information above helps to answer your question.
KC