I say 2028. Whoever will be elected in 2028 is probably between 25 and 30 right now, which are prime blogging ages. Maybe my estimate is overly optimistic though, since I suspect younger age cohorts have an even higher rate of blogging.
Of course, it may be that blogging becomes so powerful that present-day presidential hopefuls start blogging to improve their political chances, in which case the date may be even earlier -- say 2020, although that seems awfully aggressive to me. I doubt it will be much before then, however, because the older crop of aspiring presidents are already in office somewhere and locked into the current way of doing business.