Growing out short pixie cuts can be rather stressful, and make you feel very un-glamorous. Usually, people go through 3 very awkward stages; I like to call them the "I look like I need a haircut" stage, the "Not so marvelous mullet" stage, and the "I don't actually know what style my hair is now stage". Thankfully there are ways you can make growing out your hair more manageable and fun, without sending out back to your stylist to chop it all off.
#1 Grow a long pixie
If you already have a long pixie you get to skip this step (congratulations)! It may feel like you are not getting anywhere if you keep getting haircuts in order to grow your hair, but more length at the top will make it a lot easier to go into stage 2.
Also during this time feel free to have fun with it. Curl it, braid it, or accessorize (I personally love adding a Alice Band).
#2 Short/Medium Graduation
The next goal would be to get the overall length. If you wish to not have a mullet (which I'm guessing you don't) then you need to have your neck hair trimmed regularly, but make sure your stylist knows not to cut the top hair. Before you know it you'll have medium length hair, and growing out the rest is very easy.
#3 Be patient for the next few months
You are very close to having long hair, all that remains is to be patient and grow out those last few inches.
Depending on how long you want your hair to be and the condition of your ends, you may need to get regular trims to ensure that you don't have split ends.
And as easy as that you'll finally have the long hair you wished for. Remember to have fun with the in between styles, they may not be exactly what you want, but playing around may bring some new looks and fresh ideas into your life!