I would use a reference until I no longer need to, which may take a lot of drawings of faces. The best reference to start of with is from those guidelines that show human proportions. There are quite a few various guidelines, the resources of scientific nature tend to be more accurate.
I suggest to Google: 'human proportions' for the overall human figure. Then Google specific body parts such as 'human face proportions'. Print out what you prefer and keep that handy near your drawing/painting area.
I think these are all amazing faces with a lot of character, especially knowing that you used no references...I'm duly impressed!!! Keep it up--with and without references. You have a distinct voice here. That's a good thing.