Ripple carry adder doubt
The addition of each A and B can start in the first XOR gate without the carry being valid. However, propagation of the correct value to S needs the carry input valid, and then some further time for the propagation of the second XOR gate.

You simply have to add up all the gates that the signal has to pass through, in series, add up all the delays, and take the worst case, for the propagation delay of the entire circuit.