I tend to think about four months is the max. I've just checked my records and in late June I pre-ordered a Halloween anthology that contains a story by a favorite author, which releases in late October. Four months. The author has just sent around a newsletter pushing the pre-order. Would I have pre-ordered a Christmas anthology in June? I don't know. Six months is a long time. On the other hand, if the book were part of a series rather than a stand-alone, six months--with occasional newsletters containing updates--is fine. Some writers aren't that fast, and fans will wait.
If your purpose is to assure sales of the next book, in theory a shorter pre-order period is best. In reality, the time period that works best is totally dependent on how much readers like your writing, your books, or this particular series. I've done rapid release with pre-orders on a dog of a series and it confounded all the received wisdom about both tactics. If readers are hot for the next book, they're going to pre-order no matter what the lead time is.