How many days staying at my partners property would be considered cohabitation?

There is no definitive number of days that automatically establishes a couple as living together. Factors that may determine cohabitation include:

If one partner moves most of their belongings into the other’s property.

Does the couple share expenses such as rent, utilities, or food shopping?

The length of the relationship.

A person’s address for correspondence purposes.

Merely staying over occasionally is unlikely to be considered cohabitation.