Right now, top100 has breached the 1mil EP line; but top1000 is only above the 250k line.
At the rate of growth this is keeping, you could stay top1000 as long as you had maybe 100k above the line when you went AWOL (that would put you almost in top500).
If you don't have that much difference, you can try to recoup after you come back.
However, know that in this event any loss like that will almost permanently set you back a few spots if you can't burn some puddings later.
If you are below top900, I don't know if it would be easy for you to stay there. If you are closer to top500, there's no way one day of inactivity would put you out of top1000
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In guards 1, I was fighting for a spot in top300 (fluctuated between 250 and 350), and was unable to do almost anything for a whole weekend (plus a couple of days at work I couldn't attack regularly).
Came back on Monday to find myself around 1500.
This was near the end of the event, with über-Kasumi haunting everywhere (making it almost imposible to spam guards to recoup). I still managed to get back into top1000 (Although last minute attacks pushed me out into 1005 at the end).
In the first half? You shouldn't have that big problem.
Now, if this where halfway through second phase? I'd be saying something completely different.
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