You know, this is about escaping the freelancing world, but a lot of the same ideas really apply to escaping the salary job world too. Build a product w/recurring revenue, build up a customer base, eventually quit your day job.
Your employer (reasonably) has an expectation that you will give them 100% during at least 9-5 hours and also be available for overtime periods on occasion.
If you are building a product or doing consulting on the side for something with real customers (especially if they are businesses) they also probably have a reasonable expectation of being able to reach you within business hours to resolve any serious problems.
You may be able to find customers who are happy with limited evenings and weekends only support, but these are probably only going to have a small intersection with "serious" customers who are happy to drop enough cash on your product/service to make it a viable full time thing one day.
What is the best way to balance this?
A lot of consultants could go beyond just selling their time and consider selling training workshops, short ebooks or video series, retainer packages (one of my student's offers a CEO-ready monthly report to his clients, which shows the effect of his monthly tweaks on their bottom line), and so on.
Not all of these scale — you can't have 1,000 retainer packages or 1,000 students in your live workshop. But doing these things (think: pricing, not billing) helps condition you to be better at sales & marketing and coming up with a value proposition... experience that will really pay off once you decide to build that SaaS :-)
If you're consulting you can set expectations on how much time you give to clients daily.
If I give clients a focused (no HN, twitter, coffee breaks etc, pure heads-down work) five hours of effort then they will likely be happy with the results. This gives me time to work on product, do chores, play with the baby and do whatever else is important to me.
This is obviously not feasible if you're a full time employee, where the usual expectation is that you're there in person whether you're productive or not.
I was thinking more in terms of "My server died and my business is burning because of something I could fix in 10 minutes with SSH" or "my main client had their server hacked by a 0 day RoR vulnerability" and I'm stuck here filling in TPS reports until 5.
I suppose some level of monitoring and a deposit + SLA left with someone you trust to resolve first line issues could go a long way.