my HA battery lasts for about 2 weeks or so. I try and change my battery when i have a feeling that it might go dead on me during the school day or during a band rehearsal/ competition. I use 675s as well. My CI rechargeable batteries lasts for about a day but my disposable batteries lasts for about 3 days which is good because my competitions usually lasts about 1-3 days if its a super regionals or grand nats finals.
If you're too worried about your batteries going dead at the last possible minute when you're going out somewhere, I'd suggest changing your batteries every morning so they would last all day. and at night charge your rechargeable batteries overnight and you can use them again in the morning so they'll be ready for you. I do that every morning everyday before i go to school. I have to figure out which one of my rechargeable batteries are like bad so i can let my dad know i need a new rechargable. Yes it might be a hard thing to remember to do but it might help.