Each lab will define their own normal range for C-peptide.
Additionally, a single C-peptide is of limited use - you need two; one fasting C-peptide, and a subsequent C-peptide taken 1 hour after a Glucose Challenge
It is the comparison between these two numbers that is most informative.
Fasting C-Peptide is simply a measure of how much insulin was produced by your body "recently", reduced by the amount your body has metabolized. Since each person's Basal rate of C-Peptide production is different, and their rate of clearance is also different (and affected by factors such as whether or not they have done vigorous exercise recently; whether or not they are recovering from an injury; what their cortisol levels are, etc).
However, this fasting value gives a baseline against which the subsequent glucose challenge C-peptide can be assessed.
This comparison allows a relatively accurate assessment of how much insulin is produced & released in response to a glucose challenge.
If this number is "low" (especially if you also show elevated blood sugar levels)
- your pancreas simply is unable to produce enough insulin (type 2 with pancreatic burnout; type 1; type 1.5; surgical/chemical destruction of the pancreas).
If this number is "high" and your bg levels remain normal
- you have insulin resistance, but it has not progressed to "full blown" type 2 diabetes
If this number is "high" and your bg levels also become elevated
- you have Type 2 diabetes
If this number is "normal" - then look at your bg in response to the Glucose Challenge.
Assuming you are talking about "nanno grammes per millilitre" (ng/ml)
Children <15 years are expected to have a fasting reading of between 0.4 - 2.2 ng/ml
Adults are expected to have a fasting reading of between 0.4 - 2.1 ng/ml
After a Glucose Challenge, you would expect a range of between 2.0 and 4.5 ng/ml
(but for example, a fasting reading of 0.4 should not jump to 4.5 after a glucose challenge unless the person is already (becoming) diabetic, even though both numbers are "within the normal range" - this is why having both numbers to compare is a good thing.)