I tried the better laundry monitor out there and found it needing some tweaks. So I changed the logic in it to use a state machine and also made it so the user can specify a number of extra times and the delay between the times user is notified when it is finished. I also found this worked well for my dishwasher.
I also made it so it can use a Echo Speaks Device code to do a "Play Announcement All" ensure you don't max out your requests if you want all your Echos to tell you when its done.
here is the code:
Just install both as apps and you should be good to go.