I think it's 3s after IGN on, then a further 10s to delete pairings.
http://www.e38.org/ULFOwnerManual.pdf
Says
Quote:
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Originally Posted by Pairing instructions
1. Depress the send /receive (pairing) button located in the center console, for at least 1 second prior to switching on the ignition and continue to hold the button down for approximately 2-3 seconds after the ignition is turned on.
2. Release the button.
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Quote:
Deleting Devices
The paired Bluetooth mobile phones can only be deleted as a single group from the ULF control module.
The pairing procedure must first be activated, and then the send/receive
button on the installed eject-box or the pairing button located at the rear of the center console must again be depressed for an additional 10 seconds after the “Bluetooth Pairing” message is displayed on the radio in order to clear the list of paired devices from the control module and the message “paired devices deleted” is displayed.
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So maybe not the answer for you, since you can't enter pairing.