I just send BTC from my electrum wallet to Changelly so that I can convert to altcoins 8 hours ago, but I set my transaction fee to low (5 satoshi per byte), it is still zero confirmation as at right now.
What i want to know is, as long as there is one or two confirmation (doesn't matter how long), I will receive my altcoins, right? Or is there a window of payment?
Will they return me my BTC if somehow after 72 hours they still don't receive my BTC or something
Yes just wait your transaction to get confirmed and you should receive your altcoin. Estimated time for fees with 1-10 sat/byte will take 3-100 blocks. If your bitcoin transaction is still not confirmed within a day or more because of transaction overload there's a chance that your bitcoin transaction could get rejected and your coins will go back to your electrum wallet.