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having your borders secure
Secure is not a definite term though. How secure is Lithuania's border for example?
To me a bigger problem is that a certain percentage of their population in the East actually want to join Russia (possibly brainwashed by Russian propaganda I know). And you don't necessarily want to permit a member state where a future armed uprising with foreign backing can't be entirely ruled out.
Pretty sure having your borders secure is a prerequisite of NATO membership so until Russia fucks off there can't be a timetable