If you are taking testosterone cypionate, it has a half life of 8 days. If you are injecting every 2 weeks, 1/2 the dose remains after 8 days and 1/4 remains after 16 days. You can see how this steadies out, because the shot from a month ago still has an effect. After a year or so the cumulative effect makes your levels more and more constant.
Back when I was male I took T on a weekly schedule. I found taking half as much, weekly prevented the highs and lows.
After a few months the T builds up in your system, so you find running a few days late or injecting a small amount less won't have a noticeable effect.