48 Hours or 72 hours for Tallinn?

They are both short visits in my opinion, but 72 h is better than 48 h.

Also, check your spelling. The name of the Estonian capital is Tallinn.

I spent a week in Tallinn on my first visit, then returned a few years later for 3 days before visiting Riga and Vilnius...so definitely 3 days at the least, it's truly a fascinating city with a magical old town.

We recently spent 6 days there. Plenty to do. You will want 72 hours.
City core is great and fairly compact, but be sure to get to sites on the outskirts, too. It’s easy. We used Bolt which is like Uber for short rides out of the center. Fares are quite reasonable. Picked up within minutes. You dont need a car.
Very walkable core. Be sure to plan so that you only go to the Upper city once. It’s a bit of.a hike and you don;t want to be going back and forth.

I spent 5N in Tallinn, and so was able to get outside the immediate town center to visit other areas:

Balti Jaam Market
Telliskivi Creative City
Rotermann City and KGB Museum
Museum of Occupations

And still I didn't make it to:
Estonia Open Air Museum
Lahemaa National Park

They were on the plan for this August, but got sidetracked. I still hope to go back, so my vote would be for 72 hours.