Woodhaven

So, I moved recently and my apartment complex charged me a reletting fee, and one month's rent. I knew what the charges were going to be and so I told them I wanted to do a 6 month payment plan that they offered. They told me to wait for them to send an email with the amount that was owed for each month. I waited a week and hadn't heard anything and so I called and they said they were sorry and that they would send it ASAP. Another week goes by and I called and they said the same thing. I have contacted them three times and get the same response and still nothing. I don't feel it is my job to do their job and keep reminding them. Is there any kind of time limit they have before the bill expires or anything like that? I want to know because I don't want them to try and Bill me 6 months later.