You will have to wait 4 years and 9 months (as of the date of becoming a permanent resident) before you can apply unless you are married to a US citizen (then 2 years and 9 months).
This answer is for informational purposes only and should not be construed as legal advice.
There is no fastest way. You may file for naturalization up to 90 days prior to the 5th anniversary of your admission date (or 3d anniversary if you marry a US Citizen and remain married for at least 3 years).
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The fastest way is to wait 4 years and 9 months. There is no conversion as such.
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