You should take into account two things (at least) when you use Forms Auth
regarding to cookie issues.
1) Persistent cookies do not time out, so watch out for
"createPersistentCookie " argument in RedirectFromLoginPage method.
2) As the documentation specifies, if the SlidingExpiration attribute is
true (this is the default in v1.0 and false in v1.1), the timeout attribute
is a sliding value, expiring at the specified number of minutes after the
time the last request was received and be aware that the cookie is updated
when more than half the specified time has elapsed (not immediately after
the request was received).