East Java is a land unlike no other. Most regions boast one or two-star attractions but what I found on my road trip is that each day just kept getting better. The next waterfall just got bigger and bigger and the volcanoes became more and more impressive. After 10 days exploring East Java by car, I was blown away.
I would recommend 7-10 days for this itinerary. For the traveler who likes to spend two nights in one place or have a few 'rest days' cause they like to do it a bit more relaxed... 10-12 days will make the itinerary more relaxed. Any less than 8 days and you will have to remove some spots from the itinerary.