I would definitely get more than 40 hours. I can't prove it, but they probably make 40 the minimum to see who will do just the minimum. I would aim for at least 100. Some schools require 100, so if you only have 40 hours, you're limiting the schools you can apply to.
It also helps if you can volunteer in acute care, or at a hospital. It looks great on your application. You need outpatient experience for sure, but having inpatient/acute care will definitely be a bonus. Check volunteer services at your hospital and see what kind of opportunities they have.
You don't have much time, but you don't need much time either. If you volunteered 2-4 hours a week, you could easily reach 100 within a few months. It doesn't take much. It's not like aspiring PAs who need 1000 hours, and have to volunteer full time for six months.
I have about 300 hours so far (I'm still volunteering in acute care) and I've had a great response from the schools I've appliied to.
Kevin