It will take a couple of months for the second culm to appear. You can keep them in a small container probably at least until Summer, when growth will really take off. Im not sure how long it'll take to get to any appreciable size, although 5 years is a number tossed around alot. Make sure the seedling are moist and have good lighting.

Most bamboos, especially runners, prefer hot sunny locations, as they need the sun to produce energy, which they use to grow taller and grow more rhizomes. Wait for summer and they should grow much better in the natural, strong sunlight.