Advise please (on missed miscarriage)

Where do I begin.... went for my first ultrasound yesterday (I live in uk) I should of been 12+3... so excited and couldn't wait to see my baby for the first time.. only to find out he or she had stopped growing at 7 weeks!.. heartbroken doesnt even describe how I'm feeling... so that's 5+ weeks I've not known and my body showed me no signs.. I have another appointment next week to help the process... the nurse told me my emotions and finding out yesterday may trigger the process of nature...I've woken up today cramping and slightly spotting... how long should it take from now until completely miscarriaging... I'm devastated and really scared of how long this process will take