Network affiliation contracts vary from network to network and from station to station. Some stations have their affiliation agreements posted on the stations section of the FCC website: https://stations.fcc.gov. I've seen different CBS affiliation contracts that allow anywhere from 5 to maybe just over 20 hours of prime-time pre-emptions per year. This is a huge change from say 20 years ago or so, where some of the more flagrant affiliates might have pre-empted in the neighborhood of 80 to 100 hours per year.