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.