How long does a link goes down when changing speed on an interface

The switch and server are configured properly. There is an active link but because of recent flapping issues the link auto negotiate a lower speed than it should be running on.

When hard coding the new speed as outlined below the link went down for almost 3 to 5 seconds.

 
telnet switchA
conf t
interface GigabitEthernet0/21
speed 1000
end
write mem

Logs from the C2960G Cisco switch:

 
Apr 18 14:39:13.177 UTC: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/21, changed state to down
Apr 18 14:39:14.175 UTC: %LINK-3-UPDOWN: Interface GigabitEthernet0/21, changed state to down
Apr 18 14:39:16.835 UTC: %LINK-3-UPDOWN: Interface GigabitEthernet0/21, changed state to up
Apr 18 14:39:17.841 UTC: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/21, changed state to u

Status of the port from before and after the change.

 
# before
#sh interfaces status | i 0/21
Gi0/21                       connected    413        a-full a-1000 10/100/1000BaseTX

# after
#sh interfaces status | i 0/21
Gi0/21                       connected    413          full   1000 10/100/1000BaseTX