Allowable lag time?

Congrats, Diana. There are no rules, but the general consensus is "the sooner the better." Many writers advocate beginning promotional activities before publication. Don't worry about timing and just go for it.
In fact, I'd try and figure a way to use your next release to help promote the one that's already out (esp if they're related). At the same time, use the current story to promote your next book.

Cheers, and good luck!

Also, provide a link to your book in your post or your siggy (not just to your website, unless that's the only place it can be purchased).

In trade publishing there's a lag time for all sorts of reasons: reviewers need time to read the book and write their review, while publications schedule their editions a few months in advance; and trade publishers concentrate a lot of their promotional efforts into letting booksellers know about their books, so that when those books are published they arrive on the shelves (virtual or physical) in a nice big splash.

What trade publishers don't usually do is promote their books to their potential readers a long time ahead of publication. If potential readers hear of a book, like the sound of it, but can't get hold of the book when they go to their retailer they very rarely buy it: instead they forget about it and buy something else.

If you're promoting your own self-published book by all means plan your promotional efforts well in advance, and set up promotional events and so on (although remember that signings and signing tours sell very few copies especially for an unknown author). But you might be wise to schedule those events to coincide roughly with publication, and to make sure that everyone who hears of your book understands exactly how they're going to be able to get hold of it if and when they decide to buy it.