My iPhone was stolen two month ago around 16 February 2019. I made sure it was in Lost Mode in iCloud and it was immediately in offline status when I opened iCloud.
I am positive the thief switched it off. Now I am 100% certain the Find my iPhone option was turned on on my iPhone, but will the "Lost Mode" be of any help if it was not turned on on the iPhone? Would it be showing "Offline" if I did not turn on the option?
A casual thief would eventually need to bring the phone to an Apple Store or at least power it on, at which point you would be notified. Professional thieves will scrap the phone...reselling the parts that can't be identified and may never turn the phone on again.
If your phone is turned on again, the moment it makes contact with the Internet and pings Apple's servers, it will enter "Lost mode."
If your phone were not registered with iCloud/Find my iPhone, it wouldn't show up at all. Offline means the phone hasn't been seen, but is registered.
Good luck with recovering your phone, but if it has been two months, your chances are not great.
