Understanding Imagenet training
The problem with 1000 classes is the ILSVRC2012 challenge which is using a subset of the full imagenet with "only" 1000 classes. The advantage of that is that it is fixed while imagenet itself is always expanding and new classes and examples may be added. 

Training on the full ImageNet set is a very long task and will require either a very long time or a long time and a lot of computing powers (talking tens of GPUs). Although it's not short, training ILSVRC2012 can be done in one day up to a few days depending on your hardware resources and complexity of the model being training.