Subject agreement

Hi,

Somebody please shed the light upon this:

1.Compared to the number of paid holidays enjoyed by most employees in the company , three weeks of vacation seem generous.

I would be grateful if I am wrong, correct me:

" Expressions statting one amount of time, money, weight, volume, etc., are plural in form but take singular verb. "

I think this is one amount of time so it should take singular verb.

Thank you for your kindness

R.N