Told I am going to early miscarry, how long?

Hi I have been ttc for over 3 yrs and recently started IF treatment. I was just breifly blessed with a BFP on my hpt, and my beta level on 15dpo was 16. The nurse told us to be cautiously optimistic, and after taking 4 hpts(1st 3 were + last one was -) I knew it couldn't be good. My beta on monday wasn't good the nurse said so I am going to miscarry. The thing is I wont be able to talk to my RE untill our phone appt on the 30th and they didn't tell me what to expect in the mean time? How long does it take to miscarry? My lmp was 3/18 but I didn't ovulate until cd 18 so im about 20dpo now. If my levels are so low and thats how they know its not viable how many weeks/ days can this go on? Its agony knowing I was preg and its still there..:confused: