How soon after I file a request for retrieval of a priority document from the participating office (EPO, JPO, and KIPO) should I see the image in PAIR[link] definition - PAIR?

It could take several months or longer. 

In order to maximize the likelihood of successful retrieval, the PDX retrieval logic implemented by the USPTO requires the system to wait to transmit the retrieval request to the participating office until the following condition has been met:

The U.S. application into which the priority document will be placed has been forwarded to the Technology Center (i.e., PALM [link] definition - PAIR status 20 or greater).

If a first retrieval request is unsuccessful, the system will generate another request one week later.

Consequently, several months could elapse between the time an applicant files a request for priority document retrieval and the actual transmission of the request to the participating office.

USPTO FAQ - Questions and Answers (31JUL2009)