I don't know where you are located, so I'm guessing your "C30" mix is a 30mPa mix.
Obtain a strength gain curve for that particular mix from the supplier. If that's not available, you can generally predict from such references as "Design and Control of Concrete Mixes" by the PCA (in the US), or from A.M. Neville's book on the properties of concrete.
For normal weight concrete, using conventional Type I portland cement (again, a US designation since I don't know where you are located) and no admixtures that would change the properties of the hardened concrete, you can expect to obtain about 40 to 50 percent of the 28-day strength in 3 days, and about 65 to 70 percent of the 28-day strength in 7 days. If there is fly ash or GGBFS cement in the mix, the strength gain will be slower.