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.
Doesn't coinpayments lock the rate for like 6 hours? 15 minutes sounds very short especially if the transaction has to confirm within that timespan.
Now I'm even more confused. If 15 mins is short, then why they even mark this as something worth noticing on their site?
And, if 15 mins is a lot - then where the 6 hours came from? What's the deal with this time difference in hours?..