If you are struggling to get the amount of work experience in try going back to a placement at weekends. For example, I did between a half day to a full Saturday at a rescue kennels over a long period of time and this quickly added up as an addition to what I did during the school holidays.
I would try to get as much as you can and as much variety so long as it isn't impacting on your grades as these are important too. There's no magic number for what you should get.
Good luck!