IMO, if you have an interest in Greek Life, you should never "not rush at all" unless you have a compelling reason (e.g. you were convicted of some heinous crime and even then there are always extenuating circumstances, right?).*
Try doing a couple of searches on google, entering "greek chat" and "CU Boulder" in the search terms. In the recruitment forum adpiucf has a
comprehensive thread on recruitment that I recommend every PNM should read.
*NB: recently there have been a couple of late 20s PNMs posting here who went through recruitment and were released from the process fairly early on. The general consensus appears to be that (although there may be exceptions on a given campus, due to that campus's culture) PNMs who are past "traditional college age" generally do not have outcomes that result in membership in an NPC sorority.