It's very much an individual thing - two people with the same amount of experience and the same length of hiatus will take different amounts of time to get back into the swing of things.
With the voice in particular it depends what they've been up to in the mean time. If they've been singing a little at home, doing occasional vocal exercises, and keeping fairly fit, they'll probably be able to reach their previous level fairly quickly. If, on the other hand, they haven't been looking after their voice at all, been smoking & drinking heavily for example, it may take their voice longer to reach peak condition.
As with anything, there are always exceptions to these rules of thumb - but by and large I think most vocalists would be back on form after a couple of weeks of practising.