I believe it can range from "some" to "as much as is comfortable for you". I definitely think there should be a minimum of "some" affection shown between the parents so that the children can know see the love that is in their home and can know that affection is something good to give. Of course, the best way to show your child how to give and receive affection is to give it to THEM :)
In my home, me and my husband are not shy about hugging each other and kissing each other in front of Nora. We will cuddle on the couch and I'll sometimes lie on him in a hugging way while we watch something on TV. Nora does that with both of us too and we have group hugs regularly. This is the amount that feels right to us and isn't necessarily what would feel right to everyone...
Nora is actually very affectionate with hugs and kisses and cuddling and being held and sitting on my la-z-boy with me and Adam and I like it that way. We're comfy together when we're in the home and it feels full of love.