For transactions, what is the maximum amount of time that I should set the summary step's timeout?
The timeout for the summary can be an aggregate (total) of all steps in the transaction, but not necessarily so.
If you are interested in performance, then the summary timeout number should be set to the lowest acceptable number, e.g. 5000 ms (5 sec). If you are more interested in completion of the transaction, then the number should be equal to or greater than the aggregate of all steps.