Is there a maxiumum amount per month allowed by state law that a hospital can collect on a medical bill in Washington State?

No, there are no laws that regulate monthly payments on such a debt. Hospitals are usually agreeable in working with the person to establish an affordable payment arrangement. This is especially true if the hospital has been certified a trauma center and/or a county or city public hospital rather than a private facility. Many non private hospitals have programs in place to defray cost for persons that qualify for assistance. The best choice would be to contact the hospital's accounting department to discuss the matter before the account is sent to collections.