I teach adult beginners in my studio, and this is the same question they all ask at some point. The truth is, everyone is different. I understand that adults generally don't have time to practice hours a day and do not have the opportunity to play in band every day.
The best things you can do is keep your rehearsal times consistent. Try for the same time every day, and if you do have to take a day off, at least try to make a sound with the headjoint, maybe before bed or something like that.
As for low notes, there are several things you can do to help. First, make absolute sure your flute does not have a leak. If it does, low notes will be virtually impossible. Also make sure you are holding the keys down properly. Sometimes, with the low C, I have seen students allow the E-flat key (pinkey key you usually use) pushed down slightly causing a leak. And obviously, make sure you are using correct fingerings. A good fingering chart will help.
As for making sound, I have beginners of all ages play on the headjoint every day before putting the flute together for as long as a month. This insures that the proper muscle memory and placement is happening before adding the stress of remembering fingers. Remember that the air direction changes for all octaves. This is one of my favorite article I share with students is:
http://www.jennifercluff.com/tone2.htm#angle. Also make sure you are not covering too much of the tone hole.
If you are serious about learning, the best thing you can do for yourself is invest in a teacher, if you have not done so. None of it is as easy as it seems and a teacher, even if for a short time, will be able to help you move along faster and with more confidence. Good luck and welcome to the board!