I have to think the word should have been "than," a comparison. There is as much time in 10,000 years to sing as there was when we started singing. There's no less time to sing than when we first started.
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When we’ve been there ten thousand years,
Bright shining as the sun,
We've no less days to sing ***’s praise
Than when we’ve first begun.
If you look at the verse as a whole then Clive's explanation is very clear. No matter how long you sing ***'s praises, you still have an eternity to continue to do so.
I wasn't familiar with these lyrics so I