If a compund has a half-life of seven days and you inject 100mg on day 1, there will be 50mg still in your fat tissue at day 8. The rest has slowly been absorbed into your bloodstream.
The gear that is stored in your fat is still in it's esterified form and is inactive. Gear that enters your bloodstream has the ester stripped away by esterase enzymes and then the free steroid is able to bind to androgen receptors and cause gains (and side effects).
This means that gear with a longer half-life can be injected less frequently but it takes longer for the gear to "kick in". In this case kick in means build to a high enough concentration in your body that the amount you inject in one weeks gets absorbed in one week.
For something like deca, it can take 6 weeks to reach equilibrium. This means that in the early weeks, much of the gear you are injecting is going into storage. Once the cycle is over it then takes several weeks for that gear to work it's way out of your system.
Gear with a shorter half-life builds to equilibrium faster and leaves your system faster but needs to be injected more often. For instance, test prop should be injected daily while test enanthate can be injected twice weekly.