Expect long timeout seems not work

I have a TCL script to run against a device. But one of the device status changes very slowly and it usually take between 10-20 minutes. So I set the timeout to 20 minutes. When the script is running, I could aslo manually check the device status. The interesting thing is that it always get timeout wait for that status and even the status has been changed correctly. The expect_out(buffer) is empty.

set timeout 1200

expect {

    .....

     timeout {

        puts $expect_out(buffer)

     }
}

So I am wondering during so long timeout period, how often does the TCL/Expect code to check the terminal output? It seems that it has fallen to sleep and then suddenly wake up withing looking for anything.