When a domain is about to expire, you receive
several warning by email at least 1 month in
advance. Then the domain continue to be usable
for sometime after it was expired (and you get
more warning), finaly the domain is blocked for
at least 1 month before it becomes available
to anybody. So in all, once a domain expire, there 
is at least 3 more months before someone else can 
buy it.

Cheers,
Yann Stettler