Yes, there's a built in 90 day fixed limit in the earliest versions with the "new approval" mechanism. If I remember correctly it was increased in a later SP due to complaints that approving assignments could actually take more than 90 days...
Checked around:
In November 2013 we added support for a global constant named MX_MAX_EXPIRY_TIMEOUT that can contain your custom setting. So a release after this date should have support for it. Note that this is set by the dispatcher to the mcExpiryTimeout column of the approval when the approval is initialized.
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Chris
