How much time should someone take to respond to a PhD admission offer? I have received an admission offer for a PhD at a prestigious US university. However I am also currently working in another lab outside the US with a potential to also receive PhD admission there.

I think the deadline to respond to the US school is April 15 but the professor asked me politely to give an answer within a reasonable amount of time, also my former supervisor (which has no interest in the matter) suggest not to string along the US school and try to make a decision well before the deadline.

I feel it might be unethical to wait the last minute because I respect the professor and he might lose good potential candidates. On the other hand I want to wait and see if there is a concrete possibility of pursuing a PhD at this other place.
As soon as you are sure, or April 15, whichever is sooner.

You have no ethical obligation to answer before the April 15 deadline, especially if you are waiting for an offer from another department.

On the other hand, it would be nice to answer earlier if you can.  So it would be nice of you to contact your current department's admissions committee (either directly or through your lab director), tell them that you have another admission offer but would prefer them, and ask if they're likely to offer you admission.  If they haven't decided, it would be nice of you tell them about your April 15 deadline and ask them when they expect to make up their mind.

(I'm assuming that you prefer to stay in your current lab.  If you'd rather accept the pending offer, even if you got an offer from your current lab, then what are you waiting for?  If you're not sure, then what more information do you need?)