Somewhere between 5-8 minutes I'd guess. I haven't actually timed it. My computer's a reasonably competent i7 2600k quad.
Considering your play style I'm a tiny bit surprised you haven't tried GT yet. You might be interested to know that Blood Asp recently forked GT5 and added old goodies like the fusion reactor back.
Fusion in GT: unto itself the project is nothing remotely involved like your ReC fusion project, but I bet that from tree-punching to godlike power will still take you a lot longer in GT. Challenge