I'd say it depends on what else you expect from the individual or company providing you with the offsite monitoring services. Monitoring is just one aspect of what is needed. There is also the reporting requirements of the individual or company. What do you expect them to do if alerts fire. What type of response time will the individual or company have to respond to those alerts? What actions are the to take? Do they simply report them to you and you deal with the issue(s) or do they respond and deal with the issues?

Giving you a SWAG, if I were to make a very generic offer, maybe 2 hours per day, and that would actually be 7 days a week not 5 unless you don't care what happens over the weekend.

That doesn't cover responding to issues, nor include any response time to alerts.