I tend to read until I get fed up with it and then move on to another book, hoping to return to the series at a later date. The problem with that method is, is that if I am really fed up/bored with the series when I stop, there is little incentive to restart it. Or, as has happened, I've completely forgotten about the series and only go back to it years later.
However for series that I do love, I tend to devour whatever books are in print, and then let it lie fallow, letting a few new books in the series to be published, so I can really stay in that author's universe for a while once I pick up the series again. Then I'll do a re-read from the first book through the current one.
For series where there are an insane number of books already published, like the Honor Harrington series, I'll maybe restart the series at about four books before the current one. For the Liaden Universe series, I do a complete re-read of all the books (just because I really love those characters).