If there are two or more adults living in the house, another person won't make any difference no matter how long he stays.
If you get the 25% discount as the sole occupier, then it still may not make a difference in limited circumstances, explained here:
http://tinyurl.com/2oflmg
If your visitor doesn't fall in to these categories - you must notify the council within 21 days that another adult is living with you. This is assuming your visitor is not living elsewhere where he pays council tax.