Lake Baikal: how many days minimum to spend?

You need a minimum of 3-4 days at Baikal to actually feel something about it. In that time you'll manage to get to the lake, spend a couple days driving around the shore, look at the water and understand what it's all about. If your time is really limited - 2 days will somehow work, but it'll be more like just checking it off a list than an actual rest. I usually recommend a week, especially if you're coming from far away - you can hang out in Listvyanka, take a trip to Olkhon island, and just soak in the atmosphere.