How long did Smeagol have the ring until his personality split into his good former self and the evil Gollum? At the time Bilbo took the ring from him (T.A. 2941) he had the split personality. However, he still refers to the ring as "my precious" like when he first took the ring from Deagol, rather than "our".
On the other hand he already referred to himself as "us" at one occasion when he demanded the ring from Deagol and he still used plural for the time after "chasing off" the evil Gollum when accompanying Frodo and Sam in Ithilien, until being captured by Faramir in Henneth Annûn where the evil Gollum returned.