The best way to answer (unless your job is an estimator) is to say that you're not as good at (the additional task of doing their job) being an estimator, therefore it should take about ... and give an estimate that's 125 to 150% of your quick guess, or ask for additional time to make a proper estimate.
If they agree then you've been granted a luxurious amount of extra time instead of doing extra work for free.
If they question your answer then say: "Well, what do you think?".
I worked at one place where the co-owner always asked when it was his job to know, and not use it as an excuse to interrupt. I always said double.
He always questioned it and I asked him to justify his response. Only took a half dozen times and he didn't come back for any reason. Ahhh, peace.
Don't get roped into being responsible for both the completion of the project and the date.
If you are told to finish something by a certain time it's fair to outline what result they should expect but when both ends are thrust upon you then you become the fallguy whom gets the others off the hook.
Estimates shouldn't be performed for free if they will incur considerable expense for you, nor should others expect it.
If it's another employee asking does the owner want you to take time off from your assignment to field random questions? If it's a customer (and your job is not "Estimator") you run a risk of being disciplined or fired.
If you estimate short they'll come early to check or shortly after expecting it finished - you'll certainly be in trouble and unlikely to gain an extension.
If you estimate a bit long and finish on time or early you should be praised if the work is good.
Underbidding can never help you in the long run.
If it's actually your job to estimate then you need to be more cautious about accuracy, since that is what you're paid for; still a small overestimate and offering a refund always goes over better than being late and asking for more money.