As everyone else has said,it's all about what works for you. For us, it's worth it financially, although I have recently had to take on an extra day at work (not my choice) and we aren't really any better off for it. So for me, 3 days is that 'sweet spot'.

It will be different for everyone, certainly financially as there are all kinds of things which affect it, but also because only you know what it's worth to you.