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.
I can't really use a desktop wallet as I'd have to store it externally to my actual computer in one of my external hard drives. Also, the initial installation takes a very long time and, I do have a torrent to download the entire blockchain but I am waiting to get 5 hours free when I can download it and not use my computer for anything else (or use my reapberry pis with torrenting software)!
Also, I like that coinbase cover transaction fees, I do have an ethereum wallet though and the bitcoin core ready to do a fall blockchain download!