What are the longest fixed rates that were offered to you?
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.
Im surprised that this small detail does still give out some concern on some people.Im with those words that you have said that this doesnt really matter at all.
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.