Srsly, the expansion factor should not be 4 or 10 but something like 40. If you are a quick, fluent, experienced person, and if we're talking about mathematics at any level beyond cook-book/filter calculus, and if we're talking about situations with kids who're still "not dry behind the ears" mathematically, a huger expansion factor is appropriate.
Indeed, as in other answers, one should be wary of self-deceit about the mere time to write out an appropriate response.
But, more insidiously, further, even for kids who do approximately know "the right answer", there will be a huge amount of dithering. And, there will be a huge amount of no-op self-doubt, etc.
In summary, as the level of the mathematics goes up, the expansion factor goes up, hugely. It is admittedly subtle in undergrad math, depending on circumstances, prior experience, etc. But, e.g., for PhD work, I allow an expansion factor of about 1,000. To explain why this is not a joke, and why this gives perspective on undergrad exams, I seriously comment that it is my (very considered) opinion that in starter-thesis situations, "the advisor" should "see" how to do any thesis problem suggested. And for the student this should involve much confusion, much reading, etc. So, if we ask about expansion factors, ... which, !!!, include self-education, etc., _of_course_ it'll take a novice 1,000 times longer than an expert. Hopefully one's advisor is sufficiently expert so that they can do things that much faster... as dis-heartening as this might be to a naive beginner.
When we dial-back this deconstruction to undergrad stuff, the lesson still has weight...
The dumbed-down summary's rhetorical version is "whatever factor you think is good, you should multiply by 5".
(But/and think how to avoid "time constraints"... which test things that are irrelevant...)