If it repairs 1 durability every 20 seconds, You should have Plenty of time for most builds as long as you're not surrounded by hundreds of monsters trying to whack your way out.
You can use it safely as long as you do as Aegon said and once you see the little Yellow Weapon Sign in the top right corner, get to town and give it some time. I don't expect you'd see it very often.
If it Does run down to 0... Since Eth Titans that run down to 0 aren't useable anymore, even in 1.10, I doubt the Mace would still function properly. It's not a big concern though... It takes a lot to take out that much durability. You shouldn't have any problems... Feel free to make use of a great little weapon.
Even if you are constantly attacking, you will lose about 1 durability for each 30-60 seconds. That's plenty of time for your weapon to regenerate. Also, the time that you put the weapon in your stash counts too. So I don't think that it will ever break.