An exit strategy -- if you choose to include it -- is how the investor is going to get a return on their investment. The time period and amount would be sufficient if the proposed investment vehicle is a loan. It the investment vehicle is equity then the exit strategy would be if the goal was to go public, merge with a partners, be purchased by a competitor or something where the initial investors may have an opportunity to "cash out" on the increased value of the company.
The details of a potential exit strategy are certainly part of your investment overview, presentation and support material. Unless the exit strategy is part of the core value proposition of the business -- like you have a LOC from a purchaser if certain benchmarks are met -- I don't imagine it as part of an initial letters whose purpose it is to get an audience.