Elves were in Mandos for as long as either Mandos doomed for them as a punishment, or until their spirit had resolved any issues that troubled them. Feanor and his sons for example would be there for a very long time as they committed many bad deeds while they were alive. Others who didn't do anything wrong such as the Teleri killed by the Noldor during the kinslaying probablly would have been re-embodied quickly.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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