This doesn't matter. 15 minutes is really a lot, but it makes no difference actually. You can just reload the page and you will receive an updated rate.

If  a crash within those 15minutes happens (which is highly unlikely), changelly will simple refuse to make your conversion. It is a centralized service, they will look at your rates and refund you.

So it really doesn't matter, and you will not make money by using a long fixed rate.