I have been in a long term relationship with my partner for quite a while now and we got our master's together and now in the fall we will be starting our PhDs together. Since we are both in academia our situation is a little different since we both have times when we are absolutely swamped, so it's really easy to understand that and just let the other person catch up with everything they need to do. But I'm gonna try and give some advice as best I can into how we make it all work.
I think one of the biggest things for us is us trying to make sure we are on the same wake-up and bedtime routine because then that means the times surrounding those times of the day are spent together. Also we really try to make sure we spend time not working after we wake up or before we go to bed and instead enjoy our breakfast and coffee together or enjoy winding down for the night after dinner. I really think if you have the ability to be on a similar schedule as your partner then that allows you to see each other much more frequently through the day. If you aren't on the same schedule you can easily end up just barely missing the opportunity to spend a little time with your partner. I usually wake up when he does (5:30 a.m. )for his 8 a.m. classes even though I don't absolutely need to wake up at that time and I'm totally not a morning person. But this allows us to spend all that time together before we need to leave the house. Also when we are really on top of things that wake up time allows us to work out together in the mornings/meal prep (chop veggies) for that evenings dinner.
I think one of the things that helps us a lot is even if one of us is working at home we try to be in the same room as the other person (unless we do need absolute solitude) so we can still chit chat when the person working needs a break or someone's brain to pick about something. Also while chores aren't fun we try to do them together when we can so we can have the time to catch up during that. We also almost always try to cook dinner together or at the very least eat dinner at the same time. I guess all of my advice so far is to just try and spend time together, even if it is just day-to-day stuff, but I would imagine that doesn't work for those who need a break from their partner now and then (I guess my partner and I do tend to be attached at the hip a bit...) But I do recognize that doing non day-to-day stuff also really helps keep relationships thriving.
For you to be able to do bigger things outside the day-to-day activities together during grad school I think the most important thing is time management. It took me a looooong time to figure out time management during my masters, but once I did, my partner and I were definitely better for it. It did allow us to do non-day-to-day activities more frequently (i.e. going out to eat or to the movies, going fishing or hiking or other things that we enjoy doing together). I find that the best way for us to do these fun activities together was for us to plan on X date to do whatever it was we wanted to do. This allowed me to figure out what I needed to do before that date and usually meant I was very productive in getting things done so I could feel deserved in taking the break with my partner. Basically we had to prioritize spending time together and I think that is maybe one of the most important things we learned during out master's. Of course setting aside a certain date isn't exactly spontaneous and sometimes spontaneity is great for relationships. So at times we would just shirk our responsibilities (when we knew we could get away with it for a day or weekend) and just do whatever the heck we wanted to do besides work even if it just meant a day of Netflix together.
It is absolutely OKAY to not constantly be working during grad school (take some weekends off!) and it's OKAY to take unplanned breaks to have some fun! If you want to go on a vacation with your partner then make those plans and do it! While during grad school you of course have to be productive and get things done in a timely fashion, sometimes you just need to make time for other things because that will ultimately make you a more productive student. When I realized that it was okay to not work all the time, I started to treat my grad school responsibilities as more of a 9 to 5 job (I mean more realistically I was working from ~8 to 6 or 6:30 with a lunch break) and became much more productive. It meant that I needed to make the most of those hours I set aside for work and I did. It meant that I had so much more downtime to look forward and wasn't as likely to goof off at school as I was before. It meant my partner were so much happier because we had made the point to prioritize our life in addition to prioritizing our responsibilities to grad school.
I know this went a bit long but as I was writing it I came to the realization of just how much better off my partner and I were when we better managed our time and prioritized us. I think prioritizing your partner and relationship is one of the biggest things for a relationship to remain on solid ground. It allows your bond to remain strong and by making time for your partner that shows how much you care for them. I know I went a bit stream-of-conscious-y so feel free to ask me to clarify something if its unclear.
