"how far can it go?".
Apparently shares trade at 15 PE in aggregate which is still far from outrageous pricing but the granular analysis is different with tech priced outrageously and some other companies trading more reasonably.
From a time perspective, we saw a strong rally lasting 28 months from 2005 to 2007.
Here the rally is only 12 months old.
The best case is: a rally that remains vertical until end of 2016 and reaches 11,500 (40 PE) - That is the bubble scenario to factor in although it is probably not the norm.
The reality will probably be somewhere in between. Maybe all indices globally need to reach the 20 P/E mark later this year.