I have gone through a few brief 1-5 day phases of this with my current partner of 15 months. In my case, it ended up being a matter of consistency and balancing time together vs time apart. If we spent too much time together every day without having separate experiences, conversation could get tedious because we pretty much observed everything about each other's lives without needing to talk about it it. But if we spent too much time apart without setting aside quality time for each other, it got hard to stay interested in our day-to-day interactions because we didn't get to see how the little things affected the other person, like changes in mood/thought.

If your situation is like the latter then maybe you could schedule a time to call every day or every couple of days, even just for 5-10 minutes to hear each other's voice and maybe even see each other if video calling is an option. At least for me, that helps keep my partner 'real' in my mind because I can actually observe how his life is affecting him.