Unfortunately, this is one of the limitations of the current Motorola 6208 software. The RW/FF speeds and the lack of a timeline--to see where you are in a recording--are among my top two usability complaints.
I think most agree that there should be some way to easily skip ahead in 15 minute intervals as you can on other DVRs. The Tivo, for example, lets you instantly skip forward or back to "tick marks" evenly spaced throughout program, just by pressing one button on the remote:
Part of the problem is that the TVGuide software was originally designed for use on VCRs, so it tends to lack DVR-specific capabilities. I have heard talk of a new software release coming this fall that will provide some DVR enhancements. It's not clear whether this will support existing products like the 6208, or whether it will be only for newer DVRs.
A HDTV Tivo is available, but it only works with DirecTV. That's a screenshot of it above. We're not likely to see a HDTV Tivo for cable (CableCard) until next year.
Comcast plans to make the dual-tuner Motorola 6412 available in a number of markets this fall. They recently announced an agreement to use the Microsoft DVR software, which is essentially a new version of UltimateTV. It does support faster FF/RW speeds, and I think chances are good that it will offer 15-minute skip too.
Comcast could deploy this MS DVR software to the DCT6208, although I don't think that is very likely.