Answer - whatever the height of the water under your home should be the level that you see in your sump.
Things that effect what you see:
1.) The sump has openings to allow water into it besides pipes dumping into the location. - If the sump has openings and is allowing the water underneath the home to have access to the sump then you will be seeing the actual level of water under your home. If there are no holes or entry points then the water you see could be what is left over after the pump switch turns off.
2.) weather conditions and soil saturation - You will see more water in a sump location typically while there is rain coming down and when the soil reaches it's max saturation point.
3.) the height of the sump pump relative to the floor - sump pumps that are deeper than 17 inches in the ground will see more water. If you have a deep sump location you will see more water because there's more water the further down in the earth you go.
If you're sump is working properly it will react with the rise in the level of water and remove it accordingly. If you're seeing long standing water it's possible that your sump location is too deep and needs to be updated.
Having a sump location in your home will allow water to collect. Don't be surprised when you see water, but do be cautious with how the water is being removed.
Related Reading and Links
Read about the Top 6 Basement Mistakes that contractors and homeowners make.
Need answers to your Basement Waterproofing Questions?
Read previous posts about Basement Waterproofing @ Safe'n'Dry Blog
Ask Jacob A question Directly on Pioneer Basement's Help Forums!
Image of Sump pump taken by Paul Thirst @ http://www.catskillhouse.us/blog/