With direct line of sight I would say about 30ft. If you start going behind thick walls then it drops down to about 20ft. If the dropout is within 5 minutes then the watch will still show the last reading. If the dex receiver loses contact with the iPhone then the watch will show “—” after about 5 minutes of no data.
I have been able to leave both the phone and dex receiver in my kitchen (center of my house) and walk around with no major dropouts. I’ve gone up/down stairs, in the garage, in the front yard to get the mail, and in the backyard while still receiving a signal. So, all the devices do not have to be on your person 24/7.
Hope that helps.