Pretty sure most residency salaries fall in the 25% tax bracket federally. State varies. I'd say by the end of the day you end up with about 60-70% of your salary after various taxes/retirement contributions?
So a typical resident who was single with no dependents, making say 55000 in CA (one of the higher tax states), would end up with about ~73% of his money after taxes.
Gross: $55k
Federal taxes: $7.9k
Social Security: $3.4k (many residents in CA are exempt from social security due to working for the UC system, but to compensate there's a 401(a) DCP that is almost exactly the same proportion of your salary, so that ends being a wash)
Medicare: $0.8k
California: $2.4k
State disability: $0.5k
Net pay: almost exactly $40k/year, or $3.3k/month.
To go more than 30% on taxes/retirement contributions, you'd have to be voluntarily putting money towards retirement, such as for a (Roth) IRA/401(k)/403(b). It's a good idea to do so, but the vast majority of residents don't (for a number of reasons, amongst which is the fact that physicians are notoriously bad with money).
Now, whether you see that full $3.3k or not is going to be a function of whether you have to pay for benefits. In residency, all of my benefits (health/vision/dental/life/disability/whatever) were all completely "free". That is, my portion of the premium was $0, and my salary was just salary. And they fed us hot meals 5 days a week and gave us vouchers for food on the weekend. In fellowship, I have premiums towards all of the above, plus I have to pay for parking and the hospital doesn't feed us (ever), so while I'm making nominally more money here, it's actually significantly less.