How to get more recording time with a Sandisk 8GB SDHC Class 4 memory card?

I have a Panasonic Lumix DMZ-ZS3 camera and I use a Sandisk 8GB SDHC Class 4 SD card with it.

I want to shoot some videos with this camera but I’ve noticed that when I have the settings to on Rec Quality: HD, Rec Mode: Motion JPEG I could only record for about 8 minutes only.
When I change the setting from Rec Quality to VGA I’m able to record for 21 minutes.

But the thing is I want to record in HD so that I can upload in HD instead of doing the whole export settings.

So how can I get more recording time with the Rec Quality being HD? I find it quite strange, with a 2gb sd card it can record only 8 minutes and even with an 8gb sd card it’s unable to record longer than 8 minutes.