Friendly advice offered here, DON'T GET ANYMORE CORTISONE INJECTIONS IN YOUR FOOT! You said that you had three in 2 months. WOW! This will destroy your joints and this is entirely TOO MUCH TOO SOON. I don't know your Podiatrist personally, but MOST reputable doctors and foot/ankle specialists WILL NOT INJECT MORE THAN THREE IN 12 MONTHS...
It is usually evident that if the first injection doesn't provide you with some duration of relief (1-3 mnths), then it needs to be explored by a foot and ankle specialist to find out what the REAL problem is because yours seem to appear that there is an underlying problem unforeseen.
Please have your Podiatrist refer you to a reputable specialist to examine your foot, order some test, make recommendations for custom orthotics, and prescribe treatment. I went through 6 different specialists before I actually found the right one; however, too little too late. I have gone so far to have a Sympathetic Nerve Block done in February,'98 and it didn't PHASE the pain.
My only source is surgery now and I just can't agree to it yet. Maybe never after finding this post today and reading the articles here. I started out wearing Heel Cups by Smith&Nephew, hard inserts by Spenco and soft silicone inserts by Wondersorb. None of these helped at all. I feel like if I had the doctor I have now 1 year ago and had the custom inserts in the SAS shoes, thenI may be better today. They provide wonderful support while you wear them. To answer your question how long should you wait for improvement is a question everyone with PF would like to know.
Don't mean to appear as a pessimist, but treatments vary from person to person and reaction to treatment varies also.
But, please do not let any one else inject anymore cortisone in your foot. I think there will be many more here who will respond to that also. My MRI report (3-25-98)shows the joint degenerative change of the talonavicular joint as well as the subtalar joint both areas filled with fluid. Whether or not just the 3 injections contributed to the joint deterioration, remains to be seen. I wish you good luck in your search for a NEW FOOT SPECIALIST and successful treatment.