How much time do i need for this transumatran trip?

The cost will depend on your negotiating skills. Check out all of the guesthouses and pick one you like.

"Actually your idea about a day trip from Berastagi to Sipiso piso waterfall and soak in the view seems legit"

I didn't like it at all. So much garbage.

@thelawnet

I guess it will be hard to beat the spotless waterfall i saw last December in Batanta island, but the sheer size of this waterfall should be worth the effort at least from the distance of the view point!

The fastest way through the jungle if you start at the abandoned mosque (not at all recommended!) is to walk up the river for a bit, scale the almost vertical cliff on the left then follow the ridge to the top of the hill. From there descend to your left until you see a path to the right, take two right turns and follow the path down to the camping ground.

In case anyone reading is thinking of doing this (please don't) what I meant to say was follow the stream until it turns left, then climb the cliff on the right, before following the ridge to the top of the hill.

I reiterate my previous advice about hiring a guide.

Pak Mus is probably still your best bet; for lodgings as well as local information; last time I was in the area I decided on staying at 1000 Hills which is Belgium backed/owned. In house guides etc.
Sipiso Piso; after 8 years of visiting Samosir area I decided on a visit to this waterfall, I'd stayed away due to the many stories of garbage. I was pleasantly surprised to see a lack of garbage in this area..... I visited only 2 weeks ago.

Banyaks; probably a good idea deciding against these islands... my 24 day stay 2 months ago was great for myself and girlfriend; but we'd been travelling for 4 months and enjoyed the seclusion and ease of beach life.

Harau Valley should definitely be reserved; the main accommodation, Abdi Homestay, is quite popular.

Kerinci; I have to say compared to GLNP and surrounding areas, this area is fantastic.
Kersik Tua offers fantastic opportunities of not only Mt Kerinci but surrounding areas for easy hikes... farther down is the beautiful town of Sungai Penuh; the people are friendly with very little tourists getting that far down. I was there for 3 weeks 2012 and had the best time doing day trips to waterfalls; small beautiful coloured lakes and viewpoints (in particular the viewpoint that shows Kerinci lake at one end of the valley and Kerinci mountain at the other end).
3 weeks and I saw 2 other foreign tourists.
I passed by last year while motorbiking and stayed only one night; It just hadn't changed.
Ive motorbike all over Sumatra during the past years and I promise Kerinci area is worth the extra 7 hour travel ride from Padang with Sungai Penuh being but a quick hop on a bemo.

Hopefully they have fixed the garbage problem although it could be because it's Ramadan and no local tourists. Lebaran starts tomorrow and will bring mountains of garbage.... Will try to make a further recon visit in the next month or so.

@dc1

Thanx for the heads ups, very helpful. Unfortunately (or fortunately) one has to make choices.
I am already looking forward to go to banda Aceh and takengon and Leuser is just on my way further south to reach Toba so, i guess i'll keep promising and exciting kerinci for a next visit
good to hear about Sipisopiso cleanliness, i am deciding to base myself for a couple days in tongging to do low scal day hikes around the waterfall and lake's shore before samosir. I think the viewpoint from high above on the waterfall is worth it alone!

I diched Banyak altogether, mostly for lack of time. also i had enough secluded islands/beach experience so far in indonesia and want to keep this journey more "continental".

I was tempted to hop to Nias for 5 or 6 days as a convenient stopover between Medan and Padang, the island being amazingly connected with both cities with flights, but again i need to work it out tight. i will see on the ground how i manage my time.
I will make sure to phone or sms Abdi a couple weeks before when i know exactely my exact days around Harau!

BTW had anyone bought vanilla pods before around Sumatra? if yes where exactely or maybe you had seen it somewhere? I tried my luck in Minahassa country last time with zero chance however North Sulawesi being a vanilla producing region!!

I've never seen vanilla growing in Aceh or North Sumatra, but the central highlands of Aceh would be a great place to pick up some cinnamon - it grows wild in the mid-high altitude jungle there and you can often see it for sale in village markets.

From Tongging, walk clockwise around the lake. Small roads with very little traffic, isolated little villages on the lake shore, very friendly people. The counter-clockwise road is bigger, and has much more traffic.