we too have a contract with all our clients, when it comes to domestics, we are always prepared to be flexible, if sufficient notice is given ( normally a minimum of 4 weeks ) then we will allow them to a take a short break.
It depends of course, on the wording in your agreement with the client, for example, if they are fortnightly cleans and going on hols for two weeks, we would clean as normal, if weekly and going away for six weeks, then we would agree on probably an interim clean during their absence, either way, we have always found it better to come to an amicable solution with the client, rather than strictly hold them to the terms of the contract, and we've never lost one as a result !