Universities aren't looking for a specific amount of work experience, but instead will expect you to be able to draw on experience to answer questions at interview. What really is important about work experience is how you reflect on it and learn from it.
A hospital placement is brilliant for understanding the role and the challenges of a doctor. You may want to look into volunteering as I found it gave some really useful experiences to talk about at interview. Examples of this could be care homes, St John Ambulance or youth groups, which will all give you you something to talk about when talked about skills such as teamwork, empathy etc.