My background check has been saying "In Progress" since Feb 22, 2017, when we linked the account and then it changed to "We are processing your background check. We will send you a message if we need more information" when they changed the wording in the GCKey accounts in April, I think.
However, I think, at the beginning, one usually sees "We are processing your application. The next step is to conduct a background check". before they see the "We are processing your background check ...." message. Since we are not at the end of our processing here, I can only say that from what I read on the forum, it may change to "Not applicable", if I recall correctly. This might happen right before you get PPR, when they have completed all of your reviews. However, it seems that this is hit or miss and your status may not change at all, depending on whether those in charge of your file are diligent about updating your status on your account. If there are any other messages that anyone else knows about, let me know also!
BTW, our case is not usual, as I think it changes to "We are processing your background check ..." only after the app gets to the VO".