there is no set figure, but 90days is seen as a safe figure, but 9 months, well see below as you were not living together, and your asked on the set (m) form if you have lived apart outside the uk.
from
http://www.ukba.homeoffice.gov.uk/si...df?view=Binary
4.5. Further guidance
There is no specific requirement in the Rules that the entire probationary period must be spent in the United Kingdom. For example, where an applicant has spent a limited period outside of the United Kingdom in connection with his employment, this should not count against him. However, if he has spent the majority of the period overseas, there may be reason to doubt that all the requirements of the Rules have been met. Each case must be judged on its merits, taking into account reasons for travel, length of absences and whether the applicant and sponsor travelled and lived together during the time spent outside the United Kingdom. These factors will need to be considered against the requirements of the Rules.