There is a difference between a service contract and a maintenance contract.
Maintenance Contracts usually run 10-15% of your install cost on a yearly basis. I would recommend that you start the contract from day one and make it at lest a 3 year contract. If you used a good manufacture that has a 3 year warranty and provided UPS's at the DVR's you can make a good profit because you should not have any serious issue for the first 3 years. You need to exclude acts of good, vandalism, end-user error. At the end of the 3 years you should state that there needs to be a service visit to evaluate the condition of the system. I would not renew the contract without making sure the system is still in good working condition.
Service Contract:
For most warehouse systems I will do quarterly or bi-annual service to clean cameras and inspect the system. I always plan for two techs. You need to figure how many hours it will take to clean all the cameras and if you need any special equipment like lifts. Take that time and round up. So if you can do it in 6 hours you need to round up to an 8 hour day. If you leave it at 6 hours it is very unlikely a tech will get to another site to do any work so a 6 hour day should be an 8 hour billable day. $75-$125 an hour single man rate are the going rate in the market I am in.