How do you know when a poem is finished?

I keep tinkering. But at some point you have to cut it loose. This is what I’m taking to the poetry workshop tonight (changes, while small, I think are signficant):
Vermeer’s Lady Writing a Letter
she’s taken a knife to her hair, again,
sliced away those willing strands
that each day hold her captive
in the clasp of perfect pearls
she studies herself in the mirror,
in the mellow light of morning —
a golden woman besieged by shadows
chained to a string of perfect pearls
at night she dreams of rubies
crystalline and star-filled
burning shadows bloody,
crushing seas of pearls
to evanescent dust
and so she closes her door
against the burdens of moment
turns to quill and paper —
a mirror freed by sunlight
and rich ruby dreams