An initial boot, when the unit does not have an IP address, can take a minute or two. Once the unit is assigned an IP address this gets much shorter. Here's the things that go on at boot time:

1. Self test - takes just a few seconds to test the CPU, RAM and interfaces. The LED is flashing rapidly during this time.
2. BOOTP/DHCP requests - if no IP address has been permanently assigned to the unit, it will attempt to contact a BOOTP or DHCP server on the network to get a dynamic address. If no BOOTP or DHCP server is available, this will timeout in about 90 seconds. The LED is blinking at about 2 times per second during this time.
3. On some units, after self test and BOOTP/DHCP, the operating code must be decompressed from flash into RAM before it can be started. This decompression takes about 15 seconds. The LED is blinking at about 1 or 2 times per second during this time.
4. Finally, the operating code starts and reads the configuration from the flash and starts the specified services on the ports. This can take 5-15 seconds depending on the model and the complexity of the configuration. The LED is green during this time (solid or blinking).

If the unit has been permanently assigned an IP address or BOOTP/DHCP has been disabled, step 2 is skipped.