When someone says give me 5 mins, it means

they will need a minimum of 5 mins of your time.

It would be impolite to start a timer to count down a five; and it would be impractical for the person asking (seeking) for the time to start timing you (because it would be disadvantageous to the seeker).

If you are busy, politely say no. No time for now.

If you are the seeker, be polite as not to place a number on the time you are asking the person. Five min? Who are you kidding, because most of the time, it is more than 5 min of that person's time?

I would take you seriously if you start your timer and say, "now, may i ask 5 min of your time?" That is practical but still inappropriate because the person you are asking will feel bad about it.

Ever get irked by telemarketers asking for 5 min of your time? They always waste 2 mins before getting to the point, e.g. insurance, stock, investment, marketing, etc...

Last, a polite way to ask for someone's time would be, "are you free to talk?", if yes, then go right to the point especially if you notice that the "yes" was given out of courtesy.