If you had setup a passcode on that iPhone then the thief has little to no chance guessing it and getting in the iPhone. Hopefully had a complex passcode set to lock it. Do not remove the iPhone from your device list under your Apple ID. This will prevent someone else from activating it. More than likely, the person will try and reset the passcode of the iPhone and that will erase all the data on it first. After that s/he will require the Apple ID information and things will stop right at that point. Be careful about emails or texts or calls that you may get regarding your Apple ID. Some thieves will use phishing methods to extract the information out of the victim.
Long story short, if you have a good passcode in place and do not remove the device from your Apple ID then the chances are in your favor. The person will not be able to retrieve any of your data.
Axel F.