The Baseball Prospectus Injury Ledger seemed to show about 4-6 weeks for a fibula stress fracture, but there's a large variance.
https://www.sportsmedtoday.com/fibular-stress-fractures-va-214.htm
Good info in the link above. The key is rest, otherwise you run the risk of recurrance, so while the injury is likely minor, shutting it down for a week or two and being conservative on the rehab is likely prudent. Kinda similar in that regard in a lot of ways to E-Rod's injury.