An SLA must always be stored in a charged state. A topping charge should be
applied every six months (or other time intervals as recommended by the
manufacturer) to avoid the voltage from dropping below 2.10 volts per cell.

An approximate charge-level indication can be obtained by measuring the open
terminal voltage of the cell while in storage. A voltage of 2.11V reveals that
the cell has a charge of 50% and higher. If the voltage is at or above this
threshold, the battery is in good condition and only needs a full charge cycle
prior to use. If the voltage drops below 2.10V, several discharge/charge cycles
may be required to bring the battery to full performance.

(from)

http://www.allegromicro.com/techpub2.../index3311.htm

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2.11 volts per cell indicates as little as a 50% charge, according to this
source