With conventional acquisition parameters, 13C NMR spectra of liquids are rarely quantitative and usually not integrated. There are two main reasons for this. Proton decoupling is normally used to simplify the spectrum and as a way to build up sensitivity as a result of the nuclear Overhauser effect. Also, typical T1's for 13C are very long. One can still get quantitative data and a proton decoupled spectrum if a very long recycle delay (perhaps 5 minutes) is used and the 1H decoupling is applied only during the acquisition time which is very short with respect to the recycle delay. Using such parameters however makes the collection of data prohibitively time consuming. To avoid this problem, you can add a paramagnetic relaxation agent to the sample to reduce the relaxation times.
In the spectra below the data were collected with a 10 second recycle delay. In the left hand spectrum, the duty cycle of the decoupler was 100 % and the spectrum exhibits an NOE. In the middle spectrum, the NOE is greatly reduced by decoupling only during the acquisition time (decoupling duty cycle = 10 %), however the long T1's prevent the spectrum from being quantitative. The spectrum on the right was acquired similar to that of the one in the middle except some Cr(acac)3 was added to the sample to reduce the relaxation times for the carbons. This spectrum is quantitative.