Alan, as suggested it depends on the job, location etc, whether you carry parts or need to purchase parts whether you work a 24 hour shift etc.
You may want to consider different response rates to peak and off peak times, geographic locations i.e if a client is close to where you are based then a response time would be within x hours and if outside say a 10 mile radius then x hours will apply. Alternatively you could base this upon the type of repair as some have a higher priority say fixing central heating whereas a dripping tap could be a 48 hour repair etc. Normally response rates and list of priority will be discussed in an SLA or the contract before commencement. Nonetheless the key thing here is to be realistic in estimation of times it could possibly take you to respond to any works and if you cannot get there within the response times given what follow up actions are taken i.e. contact client . If no access is granted what actions do you take, whether there is a charge for no access etc., all this demonstrates quality control.
Pricing is also mentioned above and I would agree because unsociable hours tend to have a higher rates than daytime rates. Albeit response times may be quicker if working in the evenings due to less traffic.
I hope this helps