Schedule Estimates with Kanban
Utilization is less important than capacity. I would generally recommend measuring team capacity to see how soon a task can be added, rather than trying to determine the utilization of individual team members.
If you have a large number of demand-queued tasks, you may want to consider Kanban as a both a project methodology and as an estimating technique. Kanban is a great system when you need to manage (and estimate) based on work-in-progress constraints.
As a gross simplification, if you have 6 members on your team, and a work-in-progress limit of 12, you might use a trailing average of past performance to determine that your team can generally handle 36 requests in a week.
By prioritizing backlog items or measuring the total depth of your request backlog, you can estimate when task X can be pulled into the work queue, or how long it will take to "empty the bucket" of all pending tasks. Plus, the kanban itself (meaning the physical board) is a great at-a-glance visual tool.