There is no hard and fast time for repairing permissions - I have a PB running 10.5.2 and it takes almost 45 minutes, but on my intel aluminum iMac also running 10.5.2, it takes 3 minutes.
I generally get impatient when a process stalls as I track it in Activity Monitor and it doesn't do anything for several minutes. An Archive and Install can take as lttle as half-an-hour or the two hours it took you. I'm sorry there's no real answer tyo give you, but machines are variable and so are times.