How long does a 5L refill last?
That depends on hair length and how often you wash, but for most households it is many months and a great many bottles' worth.
Which scent should I choose?
Grapefruit and orange is the most popular. Seaweed and citrus suits oilier hair, jojoba drier hair, and fragrance free avoids the question.
Is it suitable for vegans?
Much of the range is, and Faith In Nature are clear about it on the bottle. Check the individual product page for the one you want.
Does it contain SLS?
The formulations are plant-based and the full ingredient list is on every product page and bottle, which is the reliable place to check rather than taking a general answer.