This chart has probably gotten more amateur premium/IC sellers into trouble than any other. A quick Google image search for "theta options decay" brought up a screen full of charts exactly like the one below. So what's wrong?
At a quick glance, it tells us that option premium decay accelerates in the last 30 days. What this, and most theta decay charts on the web fail to mention, is that this chart is specific to at-the-money options. The decay model for OTM options, which typically make up the short legs of iron condors, have a very different curve:
As you can see from the above graph (courtesy of trades4alpha.com), the OTM theta decay curve is decidedly different than that of ATM options. This significant difference drives all sorts of bad premium-selling habits, such as opening iron condors too late, holding short options longer than necessary and underestimating the gamma risks in the last 2 weeks of an option's life. Given the risk/reward ratio of iron condors, those ATM theta charts are likely the undoing of many rookie traders.