This is for my A33. The battery was replaced 200 miles ago, but my OBDII reader is still reporting "Not Ready" for "O2S", I am not getting any codes, the SES light is off and the OBDII reader says it's off as well. Everything else (that is supported) is reporting as "Ready", so everything looks good except for the O2S not being ready.

My question is, how long will it take for the O2S to be ready? Is it normal for this behavior after changing the battery?

 

Yes, it is normal. What you have done is reset the ECU by disconnecting the battery. It could take several days, even a couple hundred miles of driving for the ECU to monitor all systems and be "ready" again.