Hours have little to do with safety, either in training, or after the fact.
When you ask how many hours of training, you post "250" under the year 2010. Are you asking about the number of hours to get a basic commercial pilot certificate? Private certificate? Airline Transport Pilot certificate? Type rating?
250 hours is a generic minimum number for obtaining a commerical pilot certificate, but that's not all training, either. In fact, only a few hours of that are actual training. Do you want to know the minimum legal times to obtain a certificate, or the actual training hours that go into obtaining a particular level of certification?
It's a little more complex than simply jotting down the hours for a given level of certification.
A private pilot, for example, may require 40 hours legally, but typically has closer to 70 to 80 hours prior to taking his or her practical test. During that time, the amount of instruction received may be 20 hours, it may be more.
When completing some type ratings, three weeks of ground training, full time in a classroom, may be conducted, followed by one or two weeks of simulator sessions. More training may go into the type rating for a specific, single airplane that a pilot flies, than he or she had for all of his or her primary training. Type ratings are issued for specific types of airplanes, and pilots who hold multiple type ratings will have had considerable initial and recurrent training on each type of aircraft.
On my current airplane, for example, just the recurrent or refresher training, done regularly, is 24 hours of classroom and 12 hours of simulator, plus an additional 9 hours of briefing/debriefing. That's more training done every few months than is required for a basic private pilot certificate, and that's for one aircraft. The answers to your question really vary depending on the individual pilot and what he or she is flying, how much he or she flies, the nature of the training, the level of certification, the rules and regulations under which he or she flies, etc.
A typical professional pilot will have received training along the way as a private pilot, commercial pilot, an instrument rating, an airline transport pilot, a flight instructor, an instrument instructor, and a multi engine instructor. He or she may also have seaplane training, conventional gear (tailwheel) training, high performance training, complex training, type-specific training, as well as training in other areas such as different categories and classes of aircraft (helicopter, etc). While the ATP (airline transport pilot) certificate requires 1,500 hours minimum, only a fraction of that is required training. A professional pilot holding an ATP,however, may have well in excess of that amount only in training, to say nothing of a significantly higher total time.
I really don't think you can accurately make a correlation between minimum certificate flight time requirements, and mishap records, any more than you can reasonably draw a parallel between total time and mishap statistics. Recency of experience, experience in type, experience under flight conditions (night, instrument, etc), command experience, nature of training, type of operation, etc, all have a significant bearing and all make a big difference in the relationship of the various components of your question.
A colleague and I (neither of us are pilots) are writing a paper on training and safety. We are interested in the number of compulsory hours of training required to complete training before one has a licence. Our very basic research thus far has shown that aviation incidents have improved over the year and that tentatively this has been linked to improved training.
A review of 558 NTSB aviation incidents reports during 1983–2002 [Ref ] revealed that although the overall accident rate remained stable, the proportion involving pilot error decreased by 40% ‘from 42% in 1983–87 to 25% in 1998–2002’. Over the same period the rate of pilot errors related to poor decision making declined by 71%, accidents involving poor crew interaction declined by 68% and accidents during takeoff declined by 70%. The review concluded that these trends in reduced pilot error involving decision making and crew coordination may ‘reflect improvements in training and technological advances that facilitate good decisions’.
Your first paragraph suggests primary training toward a private pilot certificate (the first "license" or pilot certificate one generally receives), as you specifically asked about training "before one receives a license." The private pilot certificate is typically a 40 hour minimum, though some training systems have a lower requirement.
The second paragraph, quoting NTSB statistics, makes multiple references to a crew environment, in which case the pilots often both hold ATP certificates, and typically have substantial advanced training from numerous sources.
I have over 80 different aircraft in my logbook. I hold five different FAA certificates, with various ratings and so forth. I've been flying since I my teenage years, and have flown a wide variety of aircraft and types of flight operations. Every time I take off, I face the same possibility of an engine failure on takeoff that is faced by a student pilot; either it will, or it won't.
If I happen to have a fatal mishap, what will be the difference between me, and the student pilot who does the same? I'll die with more hours in my logbook. That's all.
I'm well into my career at this stage, and frankly, I still consider myself a student pilot.