The length of your experience doesn’t matter in getting to interview. You could have thousands of hours and still be rejected in favour of someone with less than a week because that’s how UCAT cutoffs work. And don’t worry about not having enough to write about because 4000 characters doesn’t stretch very far.
The key thing is really having enough to say by interview. If you set up a weekly volunteering opportunity now, you’d have a tonne of stuff to talk about by interview and something to talk about in your personal statement. Shadowing type work experience doesn’t go very far and stacking it up is not a good use of your time. There’s no minimum requirement for it and there are only so many points it can help you to evidence because it’s not something that allows you to acquire skills and only equips you with a very low level of insight into the challenges that healthcare professionals face. So don’t fret about it!
Focus on giving experiences that allow you to demonstrate the qualities and skills required in a trainee doctor and medical student. This is what they’re looking for. Someone who’s observed 1200 hours or neurosurgery is not going to impress them any more than someone who has gained leadership experience, demonstrated compassion and caring skills, and has held multiple responsibilities for other people.
And don’t neglect the small things. Doing an intensive first aid course through the Red Cross or St Johns is fairly easy to get and will equip you with lots of practical skills and scenario based learning. Get that in there as a way of demonstrating your interest in acquiring the skills to support those in need, your interest in health and so forth. And remember, it’s not what you do, it’s what you gain from it that counts.