How much work experience do I need?

It's not the amount of work experience that matters, it is what you get out of it. Getting medical work experience is difficult so every little helps, but if you only manage a few days in a GP surgery, or even just medically relevant volunteering, what matters is what you have taken away and learnt from that experience. Personal statements should not just read like a list of all the things you've done, they should show reflection and the ability to learn from what you have seen.