On food intolerance: roughly how long could gi distress occur after eating a certain food? would a lab test (e.g. bowel transit time) be useful?

Hard to tell: In the case of intolerance, the mechanism is often unknown and thus the time of onset cannot be well defined. On the other hand, most food reactions do occur within 2 hrs after ingestion. The test to confirm the cause would depend on the type of intolerance & the food item . To confirm whether one is intolerant to a food requires a food challenge test (preferably double-blind).