Well, if I could actually take holidays... it would involve checking emails every 10 min...
I try not to over promise. At the end of the day, unless you've promised users 24/7 support, then don' t panic. Let your users know, be up front with them. I think the majority of users out there understand the support environment. If they want urgent support, then it costs them.
After dealing with a lot of software vendors over 22 years or so, I've begun to realise that my support is excellent compared to the "big boys".
Besides, if you are very busy and need someone to cover you during holidays, I'm available... no seriously, I may be able to help, always happy to pick up any extra work.
I send an email to my clients stating I'll be gone from this date to that date and feel free to contact colleague X for my FMP services and colleague Y for my IT services. I tell them that I will check email occasionally, but that I prioritize family time while on vacation. X and Y almost never get contacted. Barring a server down emergency, my clients are usually willing to wait for my return. Local FMPUG groups helped me network and find X and Y.
This works for me partly because FileMaker and servers are so stable these days that urgent nees are fewer. It also works because I've culled my client list over the years. I've chosen not to work with clients who exhibited red flags on initial meetings, and jettisoned those whose money was not worth their aggravation.
I always make service agreement part of the discussion before a customer accepts my services. That way the client knows what to expect.