If you took on the tenancy after 15 January 1989, it is virtually certain that you have a short term assured tenancy. If so, these do not ever convert to anything else, no matter how long you have been in the property - they just roll-over until terminated by one or other of the parties. So the answer is, never.
If you took on the tenancy prior to this, post again - it MAY be different.
That's in England & Wales at least - I don't know what the outlying provinces do.