Peter H
My guess would be 10s of seconds from the rejection of the unfrozen pitot to the divergence in readings from the other two pitots leading to the declaration of UAS.
DozyWannabe
Nope, it's pretty much instantaneous in human terms. The software is checking and cross-checking several times a second - you'd be talking about maybe 2 or 3 seconds from initial icing-over to the systems detecting an extended discrepancy and disconnecting AP.
Sorry, surely there were two events involved, and it's the time between them we are discussing.
IIRC The first extended discrepancy resulted in the mis-diagnosis of the working pitot as faulty. The second extended discrepancy occurred when one of the two frozen pitots started to unfreeze.