I think you are rather asking a grammatical question about whether to use the verb 'need' in the question or to leave it out.
In fact you can form this question in different ways, one of them would include the verb 'to need', but since there is a verb ,there has to be at least a subject such as 'you ,one' as in :
" How long do YOU need to live abroad in order to speak fluently ?"
" How long does ONE need to live abroad in order to speak fluently?"
There are other ways and forms of asking the same question without the verb 'to need' ,where in that case another verb will be used instead and possibly the sentence structure modified as in:
" How long does it take to live abroad in order to speak English fluently ?"