How much is too much melatonin to take for a trip?

Melatonin. Since there is no established dose for melatonin, it is recommended to start low (0.3 mg) & work up to 5 mg, taken usually 30 - 60 minutes before bedtime. Potential side effects are nausea/vomiting, headache, irritability. If morning drowsiness occurs, reduce dose or take earlier before bedtime. Choose reliable manufacturer since ingredient quality may vary (not regulated by govt).
Melatonin. Melatonin usually comes in 1 mg up to 10mg (timed released) tablets. Some people respond to as little as 1 mg. 1 to 3 mg is a pretty standard dose but other people require higher. There really isn't a way to know what will be just the right dose for you. Take care.