Half a day is enough. Staying longer would involve going outside the city.
See this recent thread: https://www.tripadvisor.com/ShowTopic-g29806…
Definitely no more than one day trip from either BA or Montevideo. Once you've seen the historic centre and maybe taken a walking tour or seen a museum or two (very small) there's nothing left to do but wait for the ferry out. It's nice enough but, to be honest, I don't think it's worth the expensive trip ($75 US return from BA, one hour.)
I'm with David. We found Colonia mildly interesting, but really not worth the journey from BA. With the ferry rides, it will end up being an all-day excursion, and Colonia just did not have enough to offer to make it the best use of a day that could have been spent in Buenos Aires. The historic center is just a few blocks of small, undistinguished, and in many cases, tumbled-down, buildings. The museums were marginal at best. There was no significant architecture of the type we envisioned in a colonial city-- just modest one story buildings on cobbled streets.
If we had it to do over, we would have skipped this visit entirely.
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