I never found time to be the biggest constraint during my graduate degrees (MsC and then PhD) - but I didn’t have labs so outside of lectures I had flexibility to work from home which made it easier to fit barn time in. I did pick up and drop riding throughout but more because of field research (considerable extended travel), transportation / access, and finances and not really because of time.
Now I work a demanding job (60-65+ hrs / week), have a 9 month old, a husband with a demanding job and I still fit ~4 rides a week in. It often means I’m riding at 9 PM at night or 6 30 AM but I fit it in best I can around my other commitments.
As long as riding is helping your mental health, you can find a way to fit it in…but as soon as it becomes a drain on your energy you need to hit pause. When you have a demanding schedule you need to make sure you focus on energy-gaining activities and not further energy-draining ones!