Hey Triton, that is one of the most often asked questions on this board, but basically a couple of options:
1- get a longer ringtone
2- set your profile to allow the ringtone to 3
3- call your carrier and ask for longer ring time.
You really need to call your carrier or use the *61 (IIRC) code to increase ring time. A longer ring tone doesn't increase the time between the start of the ring and the transfer to voicemail (your call will just get transferred to the voicemail center in the middle of the ringtone).
Setting Repeat Notification to 3 or picking a long ringtone does not affect the amount of time before your calls are sent to voicemail at all.
On a GSM carrier you can use the *61 code to adjust the length (search for "8700c Ring Time Length" or use Knocka's link above) or call the carrier. You just have to be careful not to exceed 30 seconds on the *61 code or else you will get billed for calls that go unanswered and get forwarded to voicemail.