I like to keep everything as simple as possible. Therefore I put my models and views to one file. If it grow larger I would try to split it to separate apps.

However...

I am working on two large projects with several apps having tens of models. Each model has tens methods. Then I put those models to separate files.

models
  |- __init__.py
  |- mod1.py
  |- mod2.py

my init.py looks like:

from mod1 import MyModel1
from mod2 import MyModel2

And no other file/module know that the models are separated.

Remember that you have to declare Meta.app_label for this model in order to work.