They only have to be regenerated when an entity is added, or a relation has been changed.
You could also opt for custom include templates. This comes down to binding a file with your custom code to the ID: Custom_EntityAdapterTemplate in a copy of the template set config you're currently using.
If the logic is for all teh entities, simply add the code and it will end up in all the entities. If logic is for one entity only, surround the logic with: (example here, the entity is Order)
<[If StringValueEquals CurrentEntityName "Order"]>
// your code for the Order entity
<[EndIf]>
(you can also specify more than 1 entity or use NOT to exclude just one or a couple of entities. See the SDK docs for details)
then, everytime you generate your code, select that custom template set config and your custom code is generated into the entity classes.
To ease development of such templates, please use the Template Studio beta, available in the extra's section