How much distance do I need to branch from water heater?
You can choke (reduce) the hot water line as close as you like, I suppose. Thread a 3/4-1/2 reducer right in the outlet if it makes you happy.

It's not a particularly wise thing to do, however. 1/2" pipe is not a good size for more than one or two fixtures - running everything in your house that uses hot water off one 1/2" pipe will cause your hot water pressure to be excessively low when several things need hot water at once. A 3/4" pipe is more than twice as large (revisit geometry, - .19 sq in .vs .44 sq inch) so it causes less flow restriction and pressure drop - which is why the common approach is to use a 3/4" line to feed 1/2" lines, rather than 1/2" lines everywhere. 

If you'd like to branch the 1/2" lines off the 3/4" line "as soon as possible" you can certainly put multiple 1/2" branches right at the end of the 3/4 flex line. That is what many "Manifold" systems as commonly seen with PEX plumbing do.