Geography; Is 15 mm / 30 minutes a lot? If yes, can you explain why it is. If no, why not?

That depends on what you are talking about. The Earth is about 25,600 miles around at the equator, so each degree is about 25,600/360 = 71.1 miles and each minute is 71.1/60 = 1.185 miles. Then 30 minutes would be 30 x 1.185 = 35.6 miles and whether that is a little or a lot is your call.

I have no clue to what you mean by 15 mm.