The time for ADC conversion is ~ 150us, for an accurate result (without a bridge and a smothener) you need to take a least 20 samples in a semiperiod (10ms) -> a sample in 500us. You just start AD conversion and do something else for 500us or use ad interrupt and do something else in the main program. If not , use a bridge and a smoothener for the second transformer. The load (a resistor , can be the divider) for a response time can be calculated (approx) like that : time = 0,025ζ, where ζ= RC
for time = 0.5s and C = 220uf -> R = ζ/C = 40*0.5/(220*10^-6) ~ 90Kohm
(including the ADC-in divider)