From the AIC3254 applications reference guide and the stereo AGC app note:
Decay Time: defines how quickly the PGA gain is increased when the output signal level falls below the target level due to reduction in input signal level.
Therefore the time it takes for the max. AGC gain during the mute phase in the startup procedure depends both on the decay setting as well as the max AGC gain setting.
The decay setting is determined in P0R70D3:0. The Decay Rates can be programmed from 1.5625e-3dB per sample period to 4.7683e-7dB per sample period. The recommended Value of DRC Decay Rate is 2.4414e-5 dB per sample period.
You can calculate the mute time by dividing the max.gain with the DRC decay rate and multiplying the result with the inverse of the sampling rate.