I have a few tangerine volunteers in my garden (we eat a lot of tangerines and they all have lots of pits). Most citrus here (Brazil) is grafted, but in our region it's possible to find non-grafted trees, especially our orange sour lemons and tangerines (various types).
I let one get really big over about 6 or 7 years: in the past it had a lot of pest attacks, particularly those aphids that make the leaves all black, and those darn swallowtail butterflies did a job on it. Lately it is stronger after good care and a hard prune a year or two ago, but still no buds.
Everyone I ask thinks I'm totally nuts to expect fruit from this plant; I will probably cut it and replace it if it doesn't start getting with the program soon: the birds don't really use it as shelter (too thorny), the rabbits can't browse it, I need function in this tiny food forest.
How much more time should I give it? I also tried the trick that usually works (purchasing its replacement and making threats) but no results have been seen yet.
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit