I'd do them as one long day - just getting there and back takes awhile (between the journey and maybe having to wait many minutes for the vaporetto - we had upward of a 25 min wait on Murano).
We did enjoy our visit to the Murano glass museum, but even that was maybe 75-90 min (this was some years back so I can't really recall). And you could walk around for an hour popping into shops (if you want to) and maybe stumble over a glass blowing display - we went into two - one we were beckoned in to - they let us watch for 5-10 min then took us to the expensive showroom. The other just had a sign out and we went in and watched him blow a glass horse - he had a can out for tips. But if you are there after noon, I think they are generally finished then.
I thought Burano was a photographers dream, but it isn't really huge, so even 3-4 hours there would probably be enough. Torcello you could visit for an hour or two.
((I'll just share my blog entry for when my mom came with us in 2014...
We decided today to go over to Burano for a few hours. Initially we thought to go to Torcello and look around Murano as well, but hubby was having a hard time getting going, so it was almost 11am before we finally got started.
It is a bit of a long haul to get to Murano/Burano. The ferry was quite crowded from Murano to Burano. Mom got a seat, but hubby and I had to stand for the ride - I think 30-40 min. (Well, it felt like it was that long). We roamed around Burano for a bit - it was close to 1pm by the time we got there, so we found a little spot to have to lunch and then wandered a little more. Burano is now my fav of the islands. Burano - for someone like me who likes to take photos - it is a dream. The bright colours contrasting with the blue skies, the laundry hanging to dry (I think they are contractually obliged to hang out white laundry for the tourists to take photos of - lol). We were tired from the long stand and walking around, so decided to skip Torcello and Murano and just head back to Venice))