From MATS Pt.1:
If an aircraft is likely to be delayed less than 20 minutes no expected approach time is to be passed. If the pilot requests the delay he shall be informed that the delay is expected to be less than 20 minutes.
So if the controller says nothing you may still be holding for 19 minutes! Realistically though I think 99% of controllers will inform on the likely length of delay, perhaps with a phrase like "expect less than 10 minutes" or "once around the hold".