When pregnant with my dd, I decided I would nurse for 4 to 6 months, not thinking (or knowing) what she would need after that. Of course I learned more after she was born...we got to six months, so I aimed for a year. At a year, she still seemed so babyish in many ways and loved to nurse (and it was how we reconnected when I got home from work). Then we got to 18 months and I quit worrying about it....good thing, too, because she finally weaned completely at 6.5 years! I never dreamed I would be nursing a child that long, but I wouldn't change anything! I did find it interesting that the spring after dd weaned completely, she started having problems with hayfever allergies. I guess even the 10 seconds of nursing once every 3 months (her pattern for the last year or so that she nursed) was still protective for the allergies.
Ds is now 3.5 years old and still loves to nurse. I know he'll wean eventually so I rarely even think about it. We're at what I consider the really fun stage of nursing-- we might nurse once or twice in 24 hours or 10 times in 24 hours, and we can both be happy and comfortable (physically--not engorged) about it.