When time, distance, money, and weight are considered as collective units of measure, they are considered singular because they're collective. And therefore the question 'How many snacks is too many' would be answered with a collective unit of measure("-50 grams of snack is too many."), which would mean 'is' instead of 'are', because it's collective. Here are some other examples:
Four hundred snacks is more than my extended family of five weighs.
Three hours is a long time.
Fifty kilometers is too far to walk.
Five thousand dollars is too much for that ratty old fur coat.
However, when the units are considered as separate entities, they're considered plural:
"Many hours are lost in arguing over even obvious errors." (Kris's example)
Those last five kilometers were killers, each for a different reason: crocodiles, mosquitoes, rattlesnakes, robbers, and army ants.
There are fifty silver dollars in the piggy bank. (There are 50 coins....)
The last three kilograms were lost at a rate of one per week instead of one per day. (Separate units of weight)
Just remember that collective means whole and separate means some part of the whole.