The honest answer to this question is: not very much. However, this does not mean that work experience is not important - on the contrary.
Firstly, it is necessary to stress that some medical schools place great emphasis on work experience within their admissions criteria (and some graduate entrant programmes require substantial work experience before they will consider an applicant). This is a factor when should consider as far in advance as possible in order that you can organise sufficient work experience, or avoid these medical schools.
However, most medical schools are not stringent with respect to how much work experience you have, or even exactly what you have done provided that it has a bearing on skills which are important in medicine. This is partly because universities are keenly aware of how important family connections and a supportive school are in organising work experience, and do not want to discriminate on this basis.
For this reason, the most important thing about work experience is to be sure that you reflect on what you have been involved in, what you have learnt, what you have demonstrated, how you might develop in future, and how all of this relates to medicine. Being methodically reflective in this way is not easy or automatic, so it may be useful to keep some form of diary or otherwise make notes on any work experience you do.
A common question type at medicine interviews is “can you tell me about a time when you solved a problem/worked well in a team/made a mistake” and so on. This is because the more important qualities in an applicant are thoughtfulness and engagement, rather than who they have met or what they have done. (Although 6 months working with mountain rescue medics is impressive as well!)