Per Tetenal (on the US packages at least), a partially filled concentrate will last 6 months. Alessandro's recommendation to displace the air with canned air to reduce oxidation is a good one.
Tetenal E-6 concentrate keeps better than you would believe. I have used concentrate that is 9 mths since opening without any protective gas. The chemistry & densitometry guys will certainly find differences in the results, but not more than daylight changing color over a day.