A good way to tell if your sauerkraut is working well is to check for small bubbles in the sauerkraut (when using a glass jar, obviously). These should start to appear 24 to 48 hours after you have prepared your sauerkraut and stored it. If using a jar with a clamp seal, you will likely also notice bubbling around the seal - this is a good sign, it means that the bacteria are producing CO2 and the conditions in the container are becoming ideal for fermentation to occur.
You should also notice that the brine will become clearer over the same period, and if you have used red cabbage as part of a recipe, the colour will change to a more vibrant pink, as can be seen in the pictures below. The sauerkraut pictured was fermented for two weeks, but had turned pink after four days.
What you choose to do is up to you, as the flavour changes over time and some people prefer 'younger' sauerkraut than others. It is worth making a few jars at the same time and opening them at different times to see which is best for you. You can also remove some sauerkraut from a large jar and re-cover it, provided you use very clean implements and ensure the remaining sauerkraut is submerged in the brine.
What I do is allow about two to three weeks, as in my experience this has given the best results (flavour). I generally make a batch of several jars, and consume them over the course of a week or two after two weeks fermenting.
Once the sauerkraut is ready the jar can be stored in the refrigerator for several weeks (even months according to some, but I haven't yet tried this), although in this house they don't seem to last that long!