From past experience your existing template may work but will likely have very limited functionality. Everytime I've upgraded Joomla versions I've used it as an excuse to give the site a complete refresh.
Time taken varies significantly based on site size and components used. Also some components haven't been upgraded to be compatable with 2.5/3 so may pay to do an audit.
I'd definitely advise take a copy of the site using Akeeba Backup and create a test site using Akeeba Kickstart for yourself either on a local server or as a subdomain.
It's a relatively big jump. The template changes aren't that huge unless you have huge numbers of configurable options but the content and especially addons will be the hardest part.
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