Hallo Ceshoaveess84
Well, it actually depends on what you actually mean with ‘visiting Grindelwald’. If it‘s just about standing for 10 minutes in the Main Street of that (rather boring, touristic) village, then it could be done with the Rhine falls in one long day - also depending where you’re staying the nights before and after and how efficient the connections from/to those accommodations are.
But honestly, such a Grindelwald visit would not be rewarding at all. The vast majority of visitors going there do not just look at the (again, boring and touristic) village. No, they do the real thing by taking at least one of the cable cars / mountain railways to the surrounding attractions such as Männlichen, First, Kleine Scheidegg or even Jungfraujoch. Which requires spending at least(!) half a day around Grindelwald.