It all depends.... it can take 2 minutes for very simple flights... I have done that before on United.com.... or it can take hours... as you try to find flights and classes of services across several airlines in an alliance.
When we travelled to Egypt and Vienna two years ago on a first class award on United and LH, it took a call to book.... and took about 20 minutes.... not bad for a flight that would have cost over $15K!
There is no simple answer....