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Good, it is usually an indicator of good tank health. Although it should start growing on the glass soon and if you don't like the look of it you will have some extra maintenance on your hands.
I imagine it completely depends on everyone's own system. But I know in mine that after I found my first 2 pencil eraser sized spots on my glass it has spread to 30 - 40 similarly sized or larger spots all over my glass in a little over a month. Currently my reef is about 3.5 months old.