If there is no specific length of time mentioned in the lease for how long an overnight guest could stay, there is no hard and fast answer. However, if the lease states that no one may reside there but for those on the lease, then if a girlfriend stays over sufficiently often or long that she would be reasonably viewed (that is, viewed by the average, reasonable person) as having moving in, you'd be in violation of the lease and possibly subject to eviction. So if you're imaginging something more common or regular than overnights 2 - 3 times/week (e.g. weekends), you are probably taking a risk and should perhaps look for a different place to live. Otherwise, as noted above, at the point at which it becomes "obvious" to someone that you have a girlfriend living there (if it becomes obvious or if you get to that point), you could face difficulties.