Basically, you need to change the whole way you are doing this: if your hosting service restricts you to 1500 requests an hour, then that is probably 1500 requests across all of your users. so if you have 1500 users trying to chat, that's one request per hour each.
You will need to either change your hosting service (and probably pay for it, I've only seen limits like that with free accounts) or change your whole design to a "push" model rather than "pull" - probably both, since a 1500 request per hour works out at 60 * 60 / 1500 = 2.4 seconds between requests on average. Even paid for accounts are going to get pretty slow and cumbersome as user numbers rise with your current method - when I ping Google.com, the round trip time exceeds 10 ms so your whole system is going to fail because requests will only just reach the destination before you start making another, let alone access the DB and get back to your client!
Try this: open a new CMD window and type "PING nameOfYourDomain" then press enter.
ping google.com
ping codeproject.com
If the values under "time=" aren't less than 10ms (and they won't be) your system will tie itself in knots very, very quickly ...