This is one of the most thought provoking questions. There are so many factors that could alter an answer's correctness in the slightest bit. Here are the things that have changed from the beginning to the end of the movie:
1. The building/ train station that Chihiro and her family first walk through to enter the Spirit World/ abandoned theme park is red and Asian looking with its roof tiles and sign with Japanese character (letters). At the end of the end of the film when Chihiro and her parents are going to their car, the building has turned into a stone white building with much vegetation growing on it.
2. The car has a lot of leaves covering it.
3. The forest is overgrown and abundant with vegetation.
4. During the movie, if you payed close attention to the scenery and background during the night scenes, than you should have noticed the moon often changing its phases.
Here are some possible amount of days/months/years they were in the Spirit World:
A. 15-20 years (1,2,3 see above) much like slth's prediction of 19 years. You can come to this conclusion thinking that the original building was demolished and a new building was built but then soon after abandoned. But why would they leave the Ogino's car in the middle of the path?
B. 4 days, take or give one day (2 see above) You can come to this conclusion by counting the times it is light/ dark in the spirit world. Yet I don't think a forest can grow that much in 3-5 days and a new building to magically appear in place of the red one.
C. 14 days, give or take a day (4 and 2 see above) You can come to this conclusion by watching the moon phases during her time in the spirit world. But, this does not explain the overgrown forest and stone building.
D. 3-6 months (2 and maybe 3 see above) As Connybaum said this could be correct. You can come to this conclusion because the leaves could have fallen on the car in that time and maybe the forest could have grown a lot. Still, no explanation as to why a new building is present.
Now, I have always thought that Spirit World Time is MUCH slower than Material (Human, Mortal, whatever you call it) Time. I have to disagree shellybell. I think the Human world is Narnia compared to the Spirit World.
Truthfully, I don't think there is an answer to this question. I think this question is really for you yourself to decide. I hope I did help someone find their answer though.
Sources used:
http://www.onlineghibli.com/tavern/print.php?threadid=238&boardid=4&styleid=1&sid=iu9u6mi2l3ftj46apd0mg6air1&page=1 |