There was a thread about this I think a year ago on here but may have been on Sways. The consensus was 6 months to a year I think. I would say unless the owner gave you a time frame like gotta leave my board but I'll be back in a week or whatever then yeah Waterbaby's right it's yours!
Depends on the circumstances. Why was it left at your place? Do you know the owner or know who the owner is, and have you tried contacting the person either directly or through a friend? Does the owner know that you have the board? If after all that and a couple months have passed, the owner knows you have it, isn't in the hospital or out of the country, and hasn't bothered to come get it, I guess it's yours. What may happen is that the owner will then appear at your door wanting his board back