Deca is one of the most popular steroids used in bodybuildig for its mass and size gains. Deca is not known as a very "fast" builder but the slow onset and mild properties of this steroid make it more suited for cycles with a longer duration. 

In general one can expect to gain muscle weight at about half the rate of that with an equal amount of testosterone. A cycle lasting eight to twelve weeks seems to make the most sense, expecting to elicit a slow, even gain of quality mass. Although active in the body for much longer, Deca is usually injected once or twice per week. 

Deca is best stacked with any testosterone for size and mass such as Testosterone Enenthate or Sustanon however you can also stack Deca with Trenbolone or Winstrol for cutting cycles. 

The dosage for men is usually in the range of 400-800mg.

It is believed that Deca will exhibit its optimal effect (best gain/side effect ratio) at around 2mg per pound of bodyweight/weekly.