I'm going to take the opposite from the prevailing oppinion on this...

There absolutley should be a minimum amount of hours to solo (15-20),
as flying through the pattern a few times here and there is nowhere
sufficient for a pilot to solo (especially out of anything with a
tower... and forget about it in less than a certain amount of hours in
class B), this minimum shoul dbe there for the same reason that we have
a "minimum" amount of hours to get a Private, to get your instrument
rating to get a CPL. There is an amount of time that a person just
needs to be sitting behind the controls of a plane to be able to fly a
plane with reasonable ability, as they are not even proficient in
operation at that point.

An instructor that sends out a student with 3 hours or less is
definitley being reckless and endangering his students and fellow
pilots.

Taxiing out to the runway say 3 times (once every hour), having done 3
run ups (maybe), is not sufficient enough to prepare anyone for the
complex situations that may arise in the air.