<p>Does anybody use Scrum &amp; Sprint for Infrastructure.</p>

<p>I'm struggling with the concept of a Sprint that never finishes  i.e. a Network enhancement project.</p>

<p>Also any suggestions on how Item time can be built up to a Product Backlog, so that I can sanity check that resources are not overcommited on the sprint.</p>

I would suggest that you might start by refreshing your memory about the whole concept of Scrum (
http://en.wikipedia.org/wiki/Scrum
might be a good place to start).
For example I don't believe that there should be such thing as a 'never finishing sprint'. If you have some very long and/or recurring task just break it into more specific ones. Network enhancement is very generic - break it down to:
a spike to research new network equipment
a spike to review your cables layout
a task to draw the equipment physical locations and wires diagram
Estimate these and put them into your Backlog.
etc.
Then plan short (1-2) week sprints or iterations. Assign a specific goal to each of them. Add some of your tasks from the backlog to the iteration. Complete it.
Review the results, adjust the process, repeat.