It depends on whether week and month are used as nouns or as part of an adjectival phrase. For example:
The usual waiting period is about two weeks. Noun
We spent three months in France. Noun
We are going to take a two week vacation. Adjectival phrase
I have a three month contract to teach English. Adjectival phrase
You shouldn't use small numerals like 2 and 3 in your writing. It is better to spell out the numbers as I have done above.