I stretch for 6 months and get a touch up two times a year. Actually yesterday and the day before that was the end of a six month stretch.
You've already gotten some good suggestions as far as taming the area on your head that didn't take you relaxer well. Because you still have different textures on your head, I would highly recommend paying extra attention to that spot in regards to moisturizing, because if you do not, you'll probably get some breakage. Keep up on your DC's too, this will also help keep any breakage at bay. You can still do the scarf method, ponytail rollerset, etc. Mummysgirl has a thread for transitioning styles where she did a rollerset without ponytail holders on her transitioning hair (lots of new growth) and it came out great. Use the search feature to look for the thread on transitioning styles, it helps the long term stretchers too.
I really hesitate to suggest a corrective relaxer, but you might want to consider it (only on the underprocessed part). I have to think about a corrective too, because some parts on my hair came out underprocessed more than I usually like even though I texlax. HTH