30 minutes and 1 transfer. Beyond that, a transit experience degrades quickly. Some big cities with great transit still fail this test, partially due to their sheer size. For example, Beijing has a great subway system but can still feel remarkably inaccessible because the city is so large that many places worth going still take more than 30 minutes and 1 transfer. San Francisco, by comparison, has an almost non-existent subway, yet feels more accessible because of the natural constraints of the ocean and bay.
But even San Francisco isn't consistently easy to access. Compare these two maps to see how long it takes to get places by transit. Living in the center of the city means it takes at most 45 minutes to get almost anywhere. Living in the northwest means an hour or more for large and interesting parts of the city like the mission & dogpatch.