If you mean HL2RP:
If you buy a door, it is yours until the server crashes/restarts. Even if you disconnect, the door keeps owned by you.
HOWEVER, if you disconnect, the door SAYS it is owned by someone, but people are still able to buy it.
Meaning, if you disconnect and reconnect later, you will still own the door UNLESS either the map is restarted, or someone else bought it.