Travel from Rovinj to Plitvice

You'll spend half of your day in a car (7-8 hours total?). You can do it, if you think it's worthwhile. You can explore Plitvice in a good 5-6 hours. Easier to do toward summer when days are longer.

If you're traveling in or around peak season, do NOT try to do Plitvice as a day-trip. It gets absolutely mobbed to the point that your enjoyment of the park will be very seriously affected. You need to spend the night before your visit in or near the park so you can beat all the day-trippers onto the walkways. If you arrive around mid-aftermoon, it would be worth considering a 2-day pass. Someone reported here that the park wasn't too crowded in the late afternoon. I know only that it was a nightmare by late morning on a summer Friday. And I spent over an hour in the ticket line, to boot.

If you can visit on Monday-Thursday, that might help a bit, too.

I think a day trip from Rovinj to Plitvice is not a good idea. It is too far. Stay in Istria and enjoy Kamenjak Peninsula, Novigrad, Porec, Groznjan or Cres Island instead.