Depends what you mean by 'sex' and whether or not you are referring to physical or emotional satisfaction.
Couples usually 'make love' whilst singletons 'have sex'!!
There is no right or wrong answer to this.
Our sex drives are all unique to us. They will also vary at different times of our lives, as highlighted in my hub:
This focuses on cougar dating and the fact that a woman's sex drive tends to peak when she is older whilst a man's tails off. This can lead to relationship problems in couples of a similar age.
As an overarching generalisation, for me It is never too long to go without sex, if you can't be intimate with the 'right' person.