That is a very broad question, and probably won't get many replies here. There are many variables. Here are a few: type of duct, insulation used, velocity of air, turbulence, temperature of air surrounding ductwork, and probably more. If you can be more specific you may get the answers you want.
My run of HVAC-CALC tells me I am estimated to get 9% of my cooling load and 8% of my heating load through the ductwork. There. You have an actual answer to your question, although there are many reasons to be skeptical of its accuracy in your case. I used a summer design temperature of 99 degrees outside, 75 degrees inside. 114 summer grains of moisture. Winter design temperature 32 degrees outside, 72 degrees inside. My ductwork is R-6 insulated flex in an unconditioned attic. Perhaps *none* of these apply to your situation, but take it for what it's worth.
If you get Manual J calculations done using Version 8, it is claimed to do a better job of estimating heat loss thru ducts, than Version 7. And they say it is hugely more complex. The HVAC-CALC program available on this site is a great thing overall, is based on Verson 7 so it won't be the best solution available to you -- though it may be the cheapest and easiest to use.
>>and, if your ductwork is lousy, bad connections, joints and seams not sealed with mastic,
>>lots of holes in the plenums, poor, no insulation on ducts or plenums, figure on AT LEAST 20%.
Cem-bsee has a great point. This is a very big concern for every house that has ducts outside the conditioned space. Joe Lstiburek advocates ductwork inside the conditioned envelope, just to avoid this leakage problem.
I have read the average duct system has about 15% leakage, and that it can go far higher. Even Energy Star standards I believe will allow up to 5%. Absolutely if your duct system has any perceptible leaks, fixing them will be low hanging fruit for your efficiency. If you get a test such as "duct blaster" you can measure your own leakage, and a company that does such tests normally can either fix things themselves or refer to a company that does.
One advantage of flex duct is, reducing the number of joints and therefore, leakage. Normally leaks will occur at the beginning or end of a duct run but not in the middle.
My HVAC-CALC model apparently assumes zero duct leakage because there is no latent load attributed to ducts. Another reason to take my numbers with a grain of salt.