Can't say for sure because I don't know what ISP you're using.
But with mine I can pay up to 3 months late before getting a cut off notice. I know that because I once got behind on my bill.
I don't recommend you wait that long unless you have too. The late fees start adding up.
If you don't pay at all you WILL get your internet shut off until repayment and you'll have to pay a reconnection fee plus deposit to get your internet turned back on.
Trying sending them anything even $5.00 or $10.00 to keep from getting so far behind.
You can always find and sell Aluminum beverage cans if you have a scrap metal yard near you and can find some cans.
If I fill a black trash bag full of smashed cans I can get $5.00 per bag and it does add up.