In the manual it should say when to change the transmission fluid.  Many times people do wait until the car has about 90,000 miles on it.  Sometimes, if you wait too long, a gummy residue will begin to build up and then you can have problems.  On this Nissan site, there are many opinions about whether to flush the transmission fluid, and now to do it.  If this was my car, I would do the gentle drain and fill process that is recommended, and NOT the flush where the process is done by a machine with great force.  It is totally your decision, but you might want to check the car’s manual and if you don’t have one, call the service department and ask them.  There may be a way to test the fluid to see if flushing is necessary, but if the car has 90,000 miles, it probably does need to be done. http://forums.nicoclub.com/zerothread/294349.