It is never too cold. It is from November to February. Some part could get a little cold. Mumbai is however quite moderate and mildly cold during winter.
Maharashtra is worth visiting throughout the year. The climate here is moderate in some parts. The cities like Nagpur have extreme climate though. The monsoons are great as the nature blossoms to its utmost charm during this season. Plan your trip in any season and Maharashtra tourism will surely not disappoint you.However, some parts of Maharashtra has cooler weather, Pune being a good example. Summer is from April to May.