Again it's an absolute. We won't ask for an extension unless it looks like you will make it. "Turning pear shaped" at the hold shouldn't happen from an ATC point of view. (but that's what the 15 minute window is for). Taxy times for airports are published, you know how long it takes to push, you know how long it takes to taxy. Arrange to be off stand at the right time and the scenario doesn't happen.
Now - if you're running late then what you do on the phone to
Company Ops, and what they do with Flow is a different matter. Sometimes that's enough to get a new slot issued. If that is a case we get informed of the new slot, and that gives us the option to ask for an extension on top of that.
Extensions on extensions however - it isn't happening.