Peering Traffic

HI, A quick question. (Q) What percentage of the customer traffic uses the peering links in the case of a TIER - I service provider? TIER-I Servier providers can be Sprint, BBN, UUnet, AT&T and others. I know that it might depend on the service provider(ISP) individually a/c to peering policies. But an approximate percentage like 40,60,80... will be helpful. Remember that certain percentage of traffic just doesn't leave the network -or - remains within the network. IMP: Assume all the peering traffic is 100% customer traffic ONLY. BTW, the definition of "peering" being an exchange of traffic between ISPs without exchange of money. Thank you, DurgaPrasad V Gangisetti.