Couple hours after injection. Once the ester(decanoate) starts to be cleaved off the drug(nandrolone) it will be active in the blood stream. This process starts shortly after injection and as you said reaches peak blood concentration 24-48 hours after injection and then slowly declines from there over a period of weeks. Once in the blood stream it will make way to androgen receptors, bind to them, be transported to the hormone resposne element in responsive genes and alter the rate of transcription of the responsive gene; in this case the gene that is the blue print for muscle(contractile) proteins. This is increased protein synthesis and will occur once the steroid makes its way to androgen receptors which for all intents and purposes is when it is released into the blood.