Sust was first created for HRT purposes and was intended to be injected once every 7-10 days for this purpose. However, when it comes to the performance enhancing aspect of things, i.e. BB, more frequent injections are preferred.
When sust is injected once every 7-10 days, you will experience some fluctuations of your test levels. This is because of the various time released esters that make up sust. For simple HRT purposes, this is not that big of a deal...the main point of HRT is to elevate test levels beyond their natural state, and if you're on HRT then your natural levels are low. Even with the less frequent injections, and even though your test levels are fluctuating, it is doubtful that they ever reach a place to where they are lower then your natural state...make sense?
When it comes to taking testosterone for performance benefits, it is optimal to keep your test levels running smooth and even during the entire cycle. When you're training hard and if your test levels are all over the place, you will notice this major fluctuation in a negative way.