Jeslyn, if you don't have previous cycles to use as guideline, it's hard to detect as everybody's Luteal Phase can be different. But on the average based on 28 days, the LP is 14 days. usually average LP for women is between 12-16 days before your next AF, that's when your O is expected. But some people have LP that is as short as 10days, that means if your cycle is 30 days, then you O around CD20 (30-10=20).
So if you dont chart, try to use OPK, estimated for 30 days, earliest could be from CD14 onwards C18. If you use OPK, should start testing from CD10 just to play safe so you don't miss.
If just based on BDg, then BD alternate days from CD10 onwards, safest.