I book estimate jobs (more than four hours of work, estimated) on a first-come first-served basis. I usually book estimate jobs a minimum of two months in advance.
Requests from support retainer contractees (up to four hours of work, total) are also taken first-come first-served, usually within a 10 day completion timeframe. I also prioritize the requests based on time needed and urgency. Note that support retainers must be paid fully in advance before any work is completed.
Here are some examples of how I book types of jobs.