It depends on how much you use it. If the KB goes into sleep state it will help the batts last longer but still it depends on how much you use it. IMHO, if a KB's or mouse's batts last less than 3 months, it's unacceptable junk. But that's just me. I only ever owned one wireless KB (forget the brand) and that was a while ago. Batts only lasted me less than 2 weeks. I tried it 2 times and both times only 2 weeks. The second time my batts dies, the keyboard went right into the trash and I busted out my PS/2 KB and never looked back.
My logitech M10 wireless mouse lasts me as much as 6 months on the other hand, and that is acceptable. I also have a cheap, sweatshop, no-name mouse I bought from Fasttech and it also lasts many months.