I second that. As a full time student, you are allowed to work only 20 hours/week under study permit. Since you need to claim that RA is work, and claim that you do activities in addition to what is required as part of your PhD, I'd say claim at most 20 hours/week. I think there were people in the past who got away with claiming more than 20 hours but it's not safe, it might hurt you later.
As for breaking hours, I didn't do that for RA because I don't get a break as RA. I have been employed as an RA since start of PhD continuously. On the other hand, my TA contract is per term. Here is my letter as an example:
[date]
To whom it may concern,
This letter provides [name]'s detailed job description at the [univ name], to support her application for Permanent Resident status of Canada. I confirm that [name] has been working as a Graduate Research Assistant and a Graduate Teaching Assistant since [start date of PhD]. Her detailed job duties are the following.
As a Graduate Research Assistant, she assists me in research activities in the area of [research field]. [name] commenced employment as a Graduate Research Assistant on [start date of PhD]. Since then, she has been employed part time working 20 hrs/week for the past 157 weeks (3140 hours in total), at an annual salary of [amount]. Her responsibilities include:
• Conduct literature reviews, perform computational experiments, and conduct simulations. Report weekly to me via written documents.
• Assist in the preparation of grant applications.
• Supervise undergraduate summer research students in my research group.
• Compile results, attend meetings to discuss the findings, and assist in the analysis of results.
• Assist in the preparation and writing of scholarly publications for scientific journals and conferences.
• Deliver talks at conferences or meetings when required.
As a Graduate Teaching Assistant, her responsibilities include conducting tutorials to supplement lectures, assisting in the preparation and administration of examinations, and grading examinations and assignments. Her detailed hours working as a teaching assistant are listed below:
• September 7, 2010 to December 8, 2010: [#] hrs/week for [#] weeks ([#] hours in total), at an hourly salary of $[#] (+4% vacation pay).
• [3 more TA employments written similarly, not continuous]
In total, combining the Research Assistantship and Teaching Assistantship, [name] has worked 3452.5 hours in total since [start date of PhD] in addition to the time she spent towards her PhD studies at the [univ name].