I agree with twentyten.
There are no 'right' answers, and even if you spent 24 hours a day with your children, and never worked or had a life apart from them, you'd still feel guilty for other reasons - that you were stifling their development, not providing a role model which will encourage them to be aspirational etc etc
So first of all, you need to ditch the guilt - easier said than done, but try really hard to rationalise it.
To answer your question, I think most parents have a gut feeling for whether they are spending 'enough' time with their children. You know as a parent when your children are contented/settled or anxious/unhappy, and you respond accordingly, whether it's spending 30 minutes chatting, or making more radical changes to your lifestyle such as reducing your work hours.
The other key thing to remember (which its easy to forget) is that the central issue should be finding a good balance for the whole family. It's not a case of one family member being more important than another. Also, you need to differentiate between what you as a parent may feel you're missing out on, and whether this actually bothers your child. If your child is having a really happy afternoon with a childminder and other children, for instance, then feeling negatively about that is nothing to do with the child, it's about the mother perhaps needing to feel totally irraplaceable - which is clearly nonsense, as from the moment a child is born, it begins a lifetime of forming relationships.
On a practical note, when it comes to time spent, watching school plays, attending assemblies etc, well, as with many things, it's not just about quantity. As a working parent, I arranged time off to see some of the special nursery and school events which my children were involved in, but by no means all. DH managed to see some too. My children don't feel they missed out, I don't feel I missed out.....I honestly don't think they have ever counted up and thought 'Why did mum only come to 3 sports days and not 6?' Doubt they even remember! It's about establishing a positive balance overall, not spending every minute with your child. Apart from anything else, in families where one parent stays home, and one works, it doesn't necessarily follow at all that the children will remain closer to the parent at home, or want to spend time with them more - in fact it can be the opposite!
You won't find a neat answer to this one, but it sounds as though you feel in your heart you have made the right choices, and that having time apart to develop your professional life makes you feel like a better mum the rest of the time.