Could you tell me, what is the correct form?
"The most preferable length of the internship is/ are 3 months but 2 is/are also acceptable"
My intuition tells me that should be 'is' but 'are' seems to be more proper for number 3.
When numbers are used to talk about time, cost, or distance they are singular because the whole thing is taken as one concept:
The preferable length of the internship is three months.
Three hours feels like forever when you're just sitting there.
Fifty dollars is a lot for one book.
Three miles is a long way to go on foot.
"The most preferable length of the internship is three months, but two is also acceptable"
Generally, we should spell the numbers from 0-9. And I think a comma should come before the conjunction "but."