How long should the rapport-building part of an exploratory call be?

Short. Don’t spend more than a few seconds on rapport building.
Long enough to get the prospect comfortable discussing goals and challenges with you, but not so long that it requires the rest of the call to be rushed.
As long as possible. Increasing the amount of time spent building rapport will increase the prospect’s likelihood of moving to the advise phase of your inbound sales strategy.
It will vary based on your personal sales style.