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.
When it comes to conversions i dont pay much attention with this matter as long it do confirms and transferred,i dont really care afterwards.
Sorry if I'm being too noob, but what if I don't want to overpay or underpay as a result of floating exchange rate? Especially if we're talking about specific amount of coins. I just can\t understand why those details doesn't matter.