When does ORM become "requirement" for RESTful API?

I've started building a RESTful API in PHP's Slim framework. The framework appealed to me because of its light-weight design and routing features.

However, the dataset is somewhat large - there would be about 20 types of resources, with many associations and nested sets. So far, I have implemented my own custom ORM methods on resource classes, such as "select from database by primary key". Slim does not provide any ORM features. I am wondering if the scale of the API is large enough to recommend the use of a framework with built-in ORM support or integration with an ORM such as Doctrine or Propel.

Am I at the point where I should use an ORM to help mitigate the complexity of changes to database tables, attribute names, etc?

I am also open to using other technologies - it seems Python's Django is highly regarded as a framework for building RESTful APIs.