Need help to calculate time in an RC circuit with a AD converter and a multiplexer!
You need to know (or estimate) your source impedance.

Assuming you turn on the switch at \$t=0\$, at which point the cap has a voltage \$V_C(0)\$, ans the source voltage and impedance is \$V_S\$ and \$R_S\$,
the voltage \$V_C\$ across the capacitor as a function of time is:
\$V_c(t) = V_c(0) + (V_S - V_C(0))\times \left[ 1 - e^{\cfrac{-t}{C*(R_\text{mux}+R_\text{S}) }} \right]\$

The worst case time is for Vc(0)=0 and Vs=3.3V and maximum source impedance. Calculate the time at which Vc reaches 3.3V - 1 LSB and thats the time you need to wait.