So here is the general time frame for medical bills because most of the time the due date on the bill is not actually the due date. They just want money; the due date is usually just two weeks after they mailed it. I tried looking online for something like this & I haven't had luck. Please comment if you have better advice, data or additional info. I'm not very good at my job really... I have worked in medical billing for three years & I have dealt with bills for probably over half the states in the country.
One last thing, doctor's offices, billing departments &c have a lot of latitude in determining when your bill is actually 'due.' So, the timeline below is a kind of 'average' I have seen first hand. The timeline can vary greatly; I have not paid medical for close to a year without them going to collections, while others have been sent to collections after a couple months. It really depends & there is no way to tell. It's a fucked system.
So, the first question is: Do you have health insurance? If so, make sure the doctor or hospital billed your insurance. Even if it's out of network or you know they won't pay, ask to have it billed to your health insurance. The easiest way to tell if your insurance has been billed is to find the Explanation of Benefits (EOB) online or to call your insurance company and verify. At the very least, this will slow everything down by about a month.
Next, keep track of how many bills you've received for the service the hospital/ doctor is billing for. Generally, the billing dept will send out 3 invoices spaced 2 to 4 weeks apart. What this usually means is that you have about 6-12 weeks before your outstanding balance goes to collections AFTER your insurance is billed. The 'final invoice' is usually marked as such. (FYI, sometimes hospitals will split a single time you went to a hospital (date of service) into multiple bills for different doctors you saw, x-rays, or surgical center fees or whatever. Be aware that it might not all come on one single bill. If this happens, you might be dealing with one billing department OR multiple. Pay attention to what the invoices say regarding this. What this also means is you might be able to tell them two different stories...)
If you really want to slow things down (and push your luck) and you KNOW they haven't billed your insurance, wait until the final invoice & then ask them to bill your insurance... you can try the 'insurance is confusing line' on them. After that that they may go through the cycle of invoicing you three times again. They might not do that, but it's possible. If they do, you've gained another 6-12 weeks of time to pay the bill.
Also, if the invoices have your address wrong, like they forgot the apartment number or something like that, you can try to indignantly claim 'I only got this "final notice" bill.' Most billing departments will restart the invoicing cycle for this... at the very least you can buy yourself another couple weeks.
If you've reached the point, and you still cannot pay, you still have more time. After a bill ends up in collections, you usually have about a month before it affects your credit score (sometimes more, occasionally less in my experience); once it is referred to collections, you can call the collections company and ask when it will affect your credit to be certain.
When a medical bill is in collections, it usually doesn't affect your credit immediately. The collections company will usually begin charging interest immediately, though.
Overall, you probably have months before you actually NEED to pay a medical bill.... billing insurance can take a month or more, the billing dept will send multiple invoices before referring to collections, and once in collections the outstanding balance most likely will not affect your credit immediately.
Two final pieces of advice here: Medical debt usually affects your credit score less than credit card debt.
Second, it is easier to work with a doctor's office or hospital's billing department than it is to work with collections companies:
Some billing departments will offer discounts if you ask and say you can't pay; this is especially likely if you have no insurance or if your insurance didn't pay and adjust.
Some might give you more time if you call and ask for it; if you do this, I recommend not waiting until the final notice... It's really hard to know what the best strategy here is because I'm not aware of any statistics, but I think most billing departments will be annoyed if months have passed, you haven't called & you haven't paid. It really is up to them.
Most will work out payment plans, too. In my experience, a lot of payment plans require checking or a credit/ debit card... they want to be sure they will be paid; I do not know what happens if you do not have checking or banking info. I have never been in that position, so I don't have any advice.
tl;dr you probably have 2 months without insurance and 3 months with insurance before your medical bill goes to collections & then another month before it affects your credit. it's pretty fucked up