Anything from 3x to 5x is acceptable in my opinion.
Are there casinos out there that offer only 3-5x wagering requirements? I have found that bonus money and deposit bonuses are usually accompanied with at least 25x, and I have seen this number as high as 50x or 60x. 15x is the lowest I have ever seen, and I think it was advertised by a completely new casino that I wouldn't trust for other reasons.