Once IOS recognizes the interface is down, the trap is sent almost immediately.Turn on debug snmp on a router and pull a cable. You'll see the trap messages fly across the console.
It may take time for IOS to find out an interface is offline. This is an IOS mechanism and not part of the SNMP engine. Example - a T1 circuit fails, but CSU doesn't drop carrier to router. IOS has to wait for keepalive timeout to know circuit is down.Also, if there are a huge amount of trap messages, buffering does occur to keep from possibly crashing the router.
Hope this helps you out.
