We don't have a solid timeline yet, but the answer is "soon".

A lot of people have made changes to their sites in preparation for the 
upgrade, so I plan to do a refresh of the SharePoint 2013 environment 
from the current 2010 data so that these changes can be previewed.  This 
refresh will happen either tomorrow or early next week.  If there are 
not any major blocking problems identified, I will try to get the final 
upgrade scheduled for 1-2 weeks later.

If you really need to get started with SharePoint 2013 before then, I 
could provision a new site collection for you in the 2013 environment 
that will not be touched by the data refresh.  The site URL will change 
when the final upgrade occurs.

-Greg Mackinnon
ETS Systems Architecture and Administration

On 5/28/2015 12:01 PM, Lynda Howell wrote:
> What's the estimated timeline for the upgrade to SharePoint 2013? I'm 
> just starting to develop a new site with some tricky list/database 
> interactions, and I don't want to spend too much time trying to figure 
> things out in SharePoint 2010 and then have to re-figure it all for 
> 2013.  Should I just wait?  Is there an editable sandbox we can 
> practice with in the meantime?
>
> Thanks!
>
> Lynda.