I'm sorry, but your question is much to general to really answer. I am too young to even consider retirement, and I can certainly not speak for my colleagues. I am not aware of the statistics if they truly exist, so you may want to contact the APMA if they have those stats.
The amount of money someone "puts away" is an individual decision and once again can not be generalized. I know docs that make HUGE money that don't have a "pot to piss in", and I have patients that are factory workers that have put away every penny ever earned and have huge savings (they discuss it with me).
I will not discuss my personal finances with you, but I am part of a very large and successful practice. And I will tell you that your statement that "I know it seems easy to save $2 million as a doctor......" may be a little naive.
Once again, I know many SUCCESSFUL doctors that have put away more than that (well at least I think they have) and I'm confident that I know many SUCCESSFUL doctors that have not put away anywhere near that amount, even though they live responsibly.
As I stated, I believe making that statement may be a little naive. With today's expenses, the cost of a "nice home", the cost to raise children and all it's ancillary costs, the cost of college education (I'm presently paying $50,000 a year EACH for my two kids), etc., etc., adds up quickly.
I'm not complaining and consider myself fortunate. However, please worry about first getting through school, then obtaining a quality residency, then obtaining a good job, then providing quality, ethical care for your patients and providing for your family. You will then have time to prepare your finances along with an honest accountant and/or financial planner.
Worrying about putting away 2 million now may be premature and is not realistic until you have an income. And hopefully you'll do well enough that you'll double that number and still not have to live a "frugal" life.