What speed should I not exceed in low gear?

I have not checked an owners manual but it probably lists a recommendation.  I have always just listened to the RPMs and throttle feel in low range.  I try not to push mine past the RPM range I commonly hear in high range(my normal trail riding). 

[2015 Z Force 800 53/Trail EPS]

My rule of thumb is- If I can travel faster then 10mph for a sustained period, then I should be in high range not low. And as a general overall rule, no faster then 20mph or 5200 RPM in low range is always safe.