You're probably better off freezing it. At the aforementioned website they recommend only refrigerating it briefly (up to 24 hr.s). I think the fridge speeds up henna's demise relative to freezing. The fridge storage thing might merit further research though, just in case the ppl at hennaforhair are wrong.
One thing to consider too (in case you don't know): a lot of people report that henna mixtures that have been frozen produce a stronger (deeper red) dye. I assume this applies to henna mixtures that oxidized long enough for dye release. ... Just something to consider in case you want to keep your current shade of red w/out it darkening.