You can always find the latest regs here:
Electronic Code of Federal Regulations:
or google e-cfr. You want to select title 14, aeronautics & space.
§ 61.109 Aeronautical experience.
(a) For an airplane single-engine rating. Except as provided in paragraph (k) of this section, a person who applies for a private pilot certificate with an airplane category and single-engine class rating must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in the areas of operation listed in §61.107(b)(1) of this part, and the training must include at least—
(1) 3 hours of cross-country flight training in a single-engine airplane;
(2) Except as provided in §61.110 of this part, 3 hours of night flight training in a single-engine airplane that includes—
(i) One cross-country flight of over 100 nautical miles total distance; and
(ii) 10 takeoffs and 10 landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport.
(3) 3 hours of flight training in a single-engine airplane on the control and maneuvering of an airplane solely by reference to instruments, including straight and level flight, constant airspeed climbs and descents, turns to a heading, recovery from unusual flight attitudes, radio communications, and the use of navigation systems/facilities and radar services appropriate to instrument flight;
(4) 3 hours of flight training in preparation for the practical test in a single-engine airplane, which must have been performed within 60 days preceding the date of the test; and
(5) 10 hours of solo flight time in a single-engine airplane, consisting of at least—
(i) 5 hours of solo cross-country time;
(ii) One solo cross-country flight of at least 150 nautical miles total distance, with full-stop landings at a minimum of three points, and one segment of the flight consisting of a straight-line distance of at least 50 nautical miles between the takeoff and landing locations; and
(iii) Three takeoffs and three landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport with an operating control tower.
The short read on this is at least 30 hours, 20 dual, and 10 solo. It helps to achieve the minimum hours if you combine tasks, i.e. do your dual cross country at night. I did it the other way around, started with a private SEL rating and got a private rotorcraft-helicopter rating. I don't think starting from a commercial helicopter pilot rating buys you anything extra.
I found each successive private rating easier to earn as there is a lot of overlap. Airspace, radio work, navigation, weather, regulations, all are similar. The flying part isn't so bad.
-- IFMU