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Tokens generally are effective for 18 months, but there are timing considerations associated with FetchToken. Tokens may also be revoked.
Tokens are generally effective for 18 months. Expiration warnings are given seven days prior to expiration.
FetchToken timing considerations
There are two timing considerations for FetchToken.
From Getting a Token : http://developer.ebay.com/Devzone/guides/ebayfeatures/Basics/Tokens.html
When directing the user the eBay sign-in page for new token creation, the application adds the "sid" (SessionID) parameter whose value is a value defined by the GetSessionID call. Presence of the sid triggers eBay to store the SessionID along with the auth token that eBay generates.
eBay stores this pair for 48 hours, enabling the application to retrieve the auth token via FetchToken within a reasonable time after directing the user to the sign-in and consent pages. After that, the token expires and the application needs to start the process over again. If it is the first time using this process to obtain a token for the user, the application can use FetchToken to retrieve the token immediately. Otherwise, the application should wait 5-10 seconds before calling FetchToken.