How long does it take to get through clearing?

Hi,

I've been in clearing today and spoke to Queen Mary about studying Global Health and the person over the phone said I fit the requirements and that she'd be happy to offer me a place and that they'd send an email. So I got the email which said "Your application has been considered by the admissions selector and I am pleased to inform you that, subject to verification of your qualifications, grades and other details provided, we are able to offer you a place on the above course starting in September 2016." and I've added the course as a choice on UCAS Track and it now it says that the university is currently considering my application. How long does it usually take for them to confirm it? I'm really scared that I'll lose my place!