Sometimes I get it right early on, and sometimes it takes A LOT of tweaking before I'm happy with a portrait.
When I think my drawing may be finished I scan it into my computer and compare it with the original photo via Paintshop. If I'm not satisfied, I delete the scanned image and make a change. Then I scan it back in and compare the improved image with the photo.
When I counted all of the deleted images in my recycle bin, I learned just how OCD I am. I found that I adjusted the portrait of "Danielle" only 4 times after the preliminary image was sketched. "Ichabod" I tweaked 13 times, "Amelie" 32, and "Rachael" took a whopping 39 adjustments AND came with a 1:00 am bedtime!
When I finally deem a portrait complete I have to take it immediately for a coat of spray fixative so that I won't be tempted to tweak it again!