e.g. Three hours ( is – are ) a long period to wait for you.
e.g. Three thousand pounds ( was – were ) collected for charity yesterday.
When we are talking about a sum of money, a period of time or distance we consider it as one thing, and, thus, use a singular verb. Consider such amounts as lump sums rather than individual dollars (pounds, hours, etc.)
For example:
Two hours is a long period to wait for you.
Three thousand pounds was collected for charity yesterday.
Seven miles is a long way to walk home.
BUT:
If you split miles, hours, or thousands and make them distinct, you need to use a plural verb:
Three hours, one today and two tomorrow, are needed to finish this assignment.
NOTE: When we are talking about number of people, cars, etc. we use plural verb (these are not lump sums):
Three thousand people were killed.
Two thousand cars were lost in the ocean.
I hope this makes sense! 🙂
