Call your voicemail. Listen to the prompts. If you don't hear one for changing the time before pickup, use the one for additional functions.
Then set the time for 5 seconds per ring plus 2 seconds (to allow for the fact that the ring you hear and the ringback tone the caller hears aren't in sync).
The reason I can't tell you what number to press is that each carrier maintains its own voicemail server, and they change the menus from other carriers.