Facebook needs 50+ conversions to optimize your pixel.
1 day click (You need 50 conversions in 1 day to optimize)
7 day click (you need 50 conversions in 7 days to optimize)
Conversion can be VC, ATC, IC, or PUR. (Whatever you set your event for)
Always pick 7 days! unless you plan on running an ad set over $500 getting under $10 CPP.