The hard drive is only 160 GB, so you're not going to be recording very many hours of HD. If you have a 160 GB drive (most people do) you'll probably be able to record about 15 hours of HD stuff...it all depends on the bit rate that your cable company sends out. If you record a lot of the over-the-air networks (ABC, CBS, NBC, PBS, Fox) you'll be able to record less, as those tend to have higher bit rates than the cable-only channels (and therefore each recording takes up more disk space).
Time Warner has no way for you to increase the recording space, but there is a hardware hack that you can perform which will increase your recording space by 8 times. See this page and specifically post #201 for details on how to swap out your 160 GB hard drive for a 1 TB hard drive.