Good luck!
I would suggest you do have to wash and sterilise your breast pump if you are going more than a few minutes between expressing. I remember you could only use formula up to one hour after being made and I would think any milk left on the pump wouldn't be fresh.
I just googled as I thought I was probably wrong with ^ and yes, I am it says -
After Each Use
It is not necessary to sterilise the breastpump parts after each use as long as you do so at least once a day. If you are expressing milk for a sick or hospitalised baby, you will need to be more careful and we suggest that you follow more stringent cleaning instructions as advised by your health professional). Wash all parts of the breastpump that come in contact with your breast and the expressed milk in warm, soapy water. Rinse in clear water and allow the parts to air dry on a clean towel. Use a damp (not wet) cloth to wipe the motor unit of electrical pumps after each use, ensuring that the breastpump is diconnected from the mains.
Hygiene
When you are not using the breastpump, wrap all parts in a clean towel or store in a dedicated container. Wash your hands thoroughly with soap and water before touching your breast and the breastpump parts and try and avoid touching the inside of bottles and lids. Before expressing, clean your breast and the surrounding area with a damp cloth. Do not use soap or alcohol which can dry the skin and cause cracked nipples. Medela products are not designed to be sterilised in a microwave.