People who currently do not posses any type of pilot certificate

If you hold no rating at all, you will need to take 20 hours of instruction and log 20 hours of solo practice. Generally, however, it will take you closer to 45 hours of dual instruction before you are ready for your checkride, giving you closer to 65 hours total time in helicopters, if you are an average student.

Before you can take your checkride, you must pass a written exam. These days that pretty much means going to a computerised testing center. The test is multiple choice, there are study guides available that give you the answers, and passing grade is a 70. Piece of cake.

Assuming you pass your written test, and your flight instructor thinks you are ready for the checkride, you will be signed off to take a combination oral and flight test with either an FAA examiner, or more likely a Designated Examiner who is a person who does not work for the FAA, but has been designated as having enough experience to judge whether you make the grade or not.

The length of the exams are pretty much up to the examiner. Supposedly you will only be tested on subjects called out in the "Practical Test Standards" (which you should get a copy of) but in reality most examiners use that as a bare minimum and will ask you plenty of questions that are not in the PTS. One to two hours of oral exam and an hour of flying is pretty typical for the designated examiner we send most of our student pilots to.

People who currently hold an airplane certificate

First of all, as you read the regulations you should realize that you are not a "student pilot". You are not even a "student pilot in helicopters". You are a private or commercial or ATP pilot working on adding a category and class to your certificate. Thus any regulation that talks about student pilots does not apply to you. Some people want to interpret that they apply to you, but they don't.

An example would be cross country flight. As a non-student pilot, you have to receive 3 hours of flight instruction in cross country flight before you can take the checkride. However, you do not have to be "signed off" for cross country flight the way a student pilot does. Once you have been signed off for solo flight in a category and class, you can do just about anything except carry passengers. You could technically fly cross country before receiving your 3 hours of dual cross country. I know it sounds weird, but you are a rated pilot and the FAA will let you get away with a lot that a student pilot cannot.