what is the time frame if a husbands refuses to have sex with his wife that she can get a divorce based on husbands refusal of sex. in her case it has been approximately 6 years with no sexual relations.

In normal cases the husband should not avoid having sex with his wife more than 4 months. And if wife was afraid If waiting that period would cause her to be seriously disturbed and be in a critical condition then the husband must fulfill her desire and lust as needed.
If the wife loses her tolerance and would be in a seiorius critical condition, then she would be granted divorce by the religious judge.