How much lactulose can I take without overdose symptoms?

Low toxicity. Lactulose is one of the safest of all medicines. Take the dose your physician recommends, or if you are determined to experiment, until you can't stand the diarrhea any more.
I agree however. I agree with the previous answer, since most people taking lactulose stop taking it for that day, after having more than 3 bowel movements in 1 day, however electrolyte abnormalities have been reported with excessive use, especially in your sodium levels. This is very rare and usually is a result of dehydration from excessive bowel movements > 8 per day for a few days, but overall it is safe.