I think it depends how you brew it. Filter, for example, is a lot more forgiving on flavor than espresso. My decaf beans start to go bad for espresso after a few days of buying them, but for filter they will last much longer. One recommendation by coffee aficionados is to freeze your decaf beans as soon as you get them and then grind them frozen. I haven't tried that yet, I just buy in small batches so they are usually rather fresh.

Mixing half decaf with regular beans is a great idea, I've been doing that for years. The mix of flavors is good too, if you are brewing filter. You can get away with a lot more when you do filter.