Given the size of this tank, I don't think you need to, ASSUMING that you will be adding a small amount of livestock soon. If you want to wait on adding any livestock, that's perfectly OK, and adding a tiny bit of ammonia or some fish food that will decay and create ammonia will help keep that first round of bacteria alive. Either way works.
As for the time frame, there isn't one. It all depends on how fast the bacteria develops and how much they have available to consume. I set up my current 40g cube with live rock from another tank and I had no cycle at all. I added some pure ammonia one day and 24 hours later I had no ammonia and no nitrite, but my nitrate level had gone up. So the bacteria were already there. If you use new dry rocks and dead sand and don't add any bacteria from a bottle, it can easily take 2 or 3 weeks to get the process rolling and a month or 6 weeks to get to the end of the cycle.
Also, just in case you don't already know it, after the nitrogen cycle is done you aren't finished. After the cycle you have anywhere from zero to 3 or 4 blooms of diatoms, hairy algae, other algae and other bacterial blooms. These are usually self controlling. That is to say, an algae like diatoms which are hugely common in the hobby will bloom and seem like a serious problem. But 'normally' in a matter of a few days to a couple of weeks this bloom will consume all of the food it is using and will die back to a much lower and easy to deal with level. Some of these blooms can be quite hard on corals, anemones, feather dusters and other immobile critters. So it's best to wait out this 'ugly' stage before you try any corals.
