You will need three days with two nights to cross Santa Isabel Island on foot and appreciate the lovely mountainous nature, although if you are in a hurry you can make it in two days with one night (I met an old native who made it in a long full day from sunrise to sunset!). Local people will invite you to eat fruits and sleep in any village along your journey in a large hut for guests called Spirits House (often “visited” in the night by geckoes) after getting permission from the sorcerer.
You start in Buala and continue as follows:
Buala - Hiro - Hirokonga - Vara - Kolokofa - Siligodu - Kogue - Kaevanga
In Kaevanga you can cross by “canoe-stop” to amazing San Jorge Island.