Hi Priya,
An ECFMG application becomes valid immediately after applying. So for Step 1 in November, you may apply anytime before it.
I would suggest well in advance as there are a few requirements for the application that you should have beforehand while applying.
Please refer to this link before you apply. https://iwa2.ecfmg.org/appforcert/onlinedocs/afcoverview.aspx
All the best!
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