In a digital camera,how long(roughly) would a lithium ion batteries' energy last before needing charging again

is having a digital camera with a recharegeable lithium ion battery better than one with recharegeable nickel batteries ( in terms of battery power)?

This is really tough to answer. There are some cameras that work great with NiMH and others that work horribly with NiMH. Pentax has made a few cameras that take either AA (Alkaline or NiMH) or Lithium Ion. They even come with NiMH but I very rarely have seen them work well with ANY AA batteries. In these I highly recommend the Lithium Ion. They work 10 times longer. Other cameras may provide equal functionality with either NiMH or Lithium Ion.
It also greatly depends on whether or not you are using the flash a lot or the LCD is on. It also depends on whether or not you store the batteries in the camera when not using it or not. It also depends on what the camera's battery drain is (every camera will have a varying degree of battery drain whether its on or not).
This is really, really tough to answer without knowing what camera you are using.

It depends on how much you use the LCD. It is a power hog!

It also varies by camera & battery size.
lithium batteries last longer & are smaller than Ni-Cads.

Depends on your camera, the size and weight of the battery, wether you're using flash, size of the pics, If you're using LCD display or doing any manipulations on the images. They're better than Nicad, but theres no hard and fast rule. Anywhere from 100 photos to 1000's.

The good news is that most Lithium batteries don't ghost anymore, so you can feel free to whack em on charge whenever you remember- wether the battery is fully depleted or not!

One point to consider is shelf life. Lithium-ion batteries retain their charge for a long time where Ni-cads tend to lose it quickly. I've heard some of the newer AA's are getting better but Lithium is the best choice for a device that is used a while then shelved for a while.

With my camera, I get 1000 photo's with lithium ion versus 250 with ni-cad under similar shooting conditions.