Waiting two heat cycles is supposed to increase the risk of mammary cancer by 26%. The type of mammary cancer risk also changes -- with later-spayed dogs at higher risk of the most aggressive, life-threatening kind later in life. So I would opt to do it before the end of year 2, if mammary cancer is a concern to you (I've had several female GSDs rescued as seniors die of it, so it's something I take seriously).
Discussion: One study indicated a slightly increased risk of mammary cancer in female dogs after one heat cycle (8% increase), greater risk with two heats (26% increase) and increased risk with each subsequent heat.22 However, a recent systematic review of the publications that advocate neutering to reduce the risk of mammary tumors in dogs indicated that nine of 13 reports had a high risk of bias, and the remaining four had a moderate risk of bias.23 This study concluded that the evidence that neutering reduces the risk of mammary cancer is weak and does not constitute a sound basis for firm recommendations. Additionally, at the time when several of these studies were conducted (late 1960s), incidence rates for all malignant neoplasms were 453.4/100,000 female dogs. Mammary tumors accounted for half of these tumors, or 198.8/100,000. Thus, the actual overall risk at that time of any bitch getting a mammary tumor was only 0.2%.24 In any case, the figures for increased risk of mammary cancer must be compared with the 200% to 400% increased risk of other cancers in spayed females. While about 30% of mammary cancers are malignant,25 as in humans, when caught and surgically removed early, the prognosis is very good
I would not "fix" your dog. I agree with LuvShepherds. There is a very slight increase in the chance of getting mammary cancer, but there is a huge increase in many other cancers and hip, joint and ligament issues.
Also, from what my vet told me, if you catch mammary cancer early, it is not as big of a deal as it is with humans. Simple surgery, cut it out, move on with life. Hemangiosarcoma is a different story. Horrible disease that I went through just 2 years ago with my Heeler. Absolutely heartbreaking........
It's important to have a basic understanding of how a female dog's reproductive cycle works.
Every time a bitch goes into heat her body "thinks" it is pregnant for about 60 days- or the duration of what would have been a pregnancy. That means every body system (endocrine system, reproductive, nervous- brain) is preparing itself for pregnancy and birth. So ovarian cysts develop if no pups are present, oxytocin and other hormones prep for lactation-birth-mothering. It's a big deal for the bitch.
The long and short of it is that every time a bitch goes through an empty cycle, her risk of complications like ovarian cysts, pyometra, and mammary masses increases.
It is healthier for a bitch to be pregnant, than not to be pregnant, overall. An active breeding bitch generally does very well with pregnancies and is mentally and physically healthier for it (not recommending that everybody breed their dogs).
There is very little reason to leave a bitch intact if you will not be breeding her, and lots of reason not to. Go through several cycles so she is physically mature, but do some research on the risk of pyometra and mammary tumors/cancers.
Also, some thought should be given, I think, to the stress of false pregnancies on the dog's psyche.
And to answer the original question- I would recommend spaying at between 18- 24 months of age, but between cycles (2 months after her last cycle to 2 months before her next).
Don't do before 2 years old; wait until 3 if you can. The reproductive organs secrete hormones that help regulate how the dog grows and develops. Spaying too early can lead to skeletal/orthopedic issues.