I meant that it would nt be stable based on the time to confirm each block fluctuating in the network!
Okay, that makes a bit more sense. That is indeed correct: If a sudden drop in the hashrate does occur, then the usual '10 minutes on average' would not be true until the next difficulty adjustement.
But, I don't set my own transaction fees as I use coinbase who set their own.
I highly recommend ditching Coinbase for a desktop wallet.