"...How much time (hours) do you think a competent java programmer..."
- 1 night (2 to 4 hours) to learn Ruby, i don't know about "a certified one", but for me it took this time.
- 1 night (2 to 4 hours) to learn Rails.
- 1 night for a small project.
----------------------
sum = 3 nights to become a Ruby on rails developer
"....are the most difficult for him to address..."
i think the syntax itself. you can't predict which type of method or property name is going to be used. more over it has virtual property support. you can't get any clue what are the properties of a domain model object. unless you go through the database table.
for example:
-------------------
class Category < ActiveRecord::Base
end
you can't predict, what are the "property" inside "Category" object.
more usually in java:
-------------------
class Category {
private
String name;
// setter getter
}
you can get the hints.
NOTE: this is not a limitation, it is an excellent feature. it helps a lot to adopt any changes from your real database table.
best wishes for your Rails journey ...