How long

[Joseph ma]

'How long should it be?'.
"How long should it be?".

Please tell me which one is grammatically correct?

 

[5jj]

Both are grammatical.

 

[Joseph ma]

Both are grammatical.

Sorry, should I have used punctually instead of grammatically?

 

[jutfrank]

Yes—you're asking about punctuation, not grammar.

Are you trying to quote someone's speech? If so, it's okay to use either single or double marks, but I would suggest you use the double ones.

 

[SoothingDave]

Double quotes are used for quotation in American English. The single quotes are used if quoting a quote within a quote.