Is there a default casting time for a spell or class ability if none is specified?

I’m relatively new to D&D. Only been in a group for about 2 months, and still learning stuff every time we meet. But my question is, If a spell/cantrip/class ability does not specify the required casting time (i.e. action, bonus action, reaction), what is the default setting for casting?

I was just wondering if such a thing even exists. Because I was looking at class homebrews for an upcoming campaign, and some of them don’t specify how to use the class ability in combat. I could just ask my DM how they would want to go about it, but I was curious whether any such rule exists.

Is there a default casting time for a spell or class ability if none is specified?