You know, I never get why people say that. What determines how quickly a drug builds up in your system is it's Tmax, ei.: it's half life in the absorption phase. And this has nothing to do with it's terminal half life, which in the case of deca is definitely to long for people who cycle.
It's often tooted that it takes 4 to 5 weeks for test to start working. Peak blood test levels are app 12h after you pinned. And after 4 weeks what you feel is the build up of DHT, that takes time to convert from test, and the muscle glycogen, fullness. But the anabolic nature of test, ... that starts happening from the first minute you pined.
I know you probably know all this, it's really a rant on my part ...