What version are you working in? If you are using 6.0.4, you can add a Custom Expression column on the Format Results page.

Click Custom Expression button (at the bottom of the columns). Set the artifact dropdown to your Work Item/Task, and select your attribute, then click the arrow button to show it in the Custom Expression box at top right. Then append "/3600000"  (without the "") to simply convert ms to hours. Set Show As if desired (integer, test, decimal).
There is a video that shows how to add Custom Expressions; it uses the DW and SQL, but shows you the UI anyways.

If you are using an earlier version, then you will need to edit the SPARQL in the Advanced section, which means you will have to all future work in the SPARQL and might lose formatting and other options. If you do edit the custom SPARQL, here's the statement that I found in the Advanced section after adding the custom expression:

BIND(?rtc_cm_WorkItem1_estimate/3600000 as ?rtc_cm_WorkItem1_estimate_CUSTOM)

You would then include the ?rtc_cm_WorkItem1_estimate_CUSTOM in your SELECT statement.