Depends on what the couple is comfortable with, no? It's not a consistent thing I think, there are times when once every few days are good enough, and sometimes it could be every day. The key I think is communication and the couple's expectations in the relationship. If we're on the same page, it shouldn't feel like a chore. My gf and I wish each other good morning and good night every day, it's a simple loving gesture that we show each other. And it takes no more than 1 minute anyway, surely our s/o is important enough to spare a minute a day. Anything that happens in between is flexible. We still have our own obligations to attend to, and not everyone feels like talking all the time, as long as we communicate beforehand, that's fine already.