I have had a friend that in total breastfed for about 5 years (non-stop). Her babies were born around 18 months apart each, and she was nursing all the time from when #1 was born, #2 conceived-born, #3 conceived-born. When #3 was born, #1 was not very keen on nursing and stopped on her own. So, it can be done. However, my friend was very selective about her diet, eating a lot of high protein diet, fatty fish, fresh fruits and vegetables, etc to make sure that the one she is conceiving and the one she is nursing is getting enough nutrients.
As for some others, their babies automatically refused to nurse because the milk tastes different!