On desktops, long forms are pretty normal. We've all seen registration pages and e-commerce ordering processes. The longer the form is, the greater the tendency to try to break them up into smaller, more logical pieces. This is usually done in a couple of ways:
Leave it as a full page but inject enough whitespace and grouping that it doesn't look quite so intimidating
Either physically break the form into multiple pages or use show/hide techniques to accomplish the same thing
Neither of these approaches makes a lot of difference with regards to task completion. Either way, both methods are particularly unfavorable strategies within the constraints of mobile. The best things we can do to increase success are:
Completely eliminate all optional fields
Reduce the number of required fields as much as possible (get vicious about it)
Pre-fill elements with reasonable defaults
Validate fields immediately instead of waiting till the end
Give the user upfront notice about how long...