How much of an interruption is permitted between a mitzvah blessing and the mitzvah?
I was taught that we shouldn't make an interruption between saying a mitzvah b'racha and taking the action. I don't have a source for this other than that it makes sense; you don't want to risk making a blessing in vain if you interrupt with something else and never get back to it, and you should be maintaining the intention throughout, without distractions. I have two related questions: delays and interruptions.
First, is any interruption a problem or is there some minimum in terms of either time or what it is? If the person called to the torah makes the blessing and the reader announces a verse number before reading, is that a problematic interruption? If a person makes the opening haftarah blessing and then says a couple sentences about the reading to provide context before starting, is that a problem? How much interruption is too much?
Second, if there is no actual intervening activity (like introducing the portion), is there a length of delay that causes a problem? The reader can take a breath, the person making motzi can cut or tear the bread after finishing the blessing (on the way to eating), and in a group havdalah you might have to wait for the spice box to be passed to you. So the answer isn't "zero delay", but (if I learned correctly), an excessive delay is not ok. Is this a matter of judgement ("excessive" is based on the current context), or -- like the matzah at the Pesach seder that we have to eat within a specified number of minutes -- is there a halachic time limit?
For either, does it matter if the interruption was not reasonably foreseeable or something you could have planned for?
(I'm asking this question as a teacher, to help me judge when I need to be urging more of a sense of urgency, not as someone looking for excuses to delay.)