I made about $52,000 last year, and paid over $8,000 in federal income taxes by the time exemptions and deductions were figured in. (I had over $10,000 withheld, so I got a $2,000 refund.)
every other question depends on how much you have withheld... but i'll let you in on a secret:
if you have so much withheld that you get a refund more than about $4,000 (not even $40k), the IRS is likely to hit you with a penalty.
if you have so little withheld that you end up owing more than $3,000 the IRS WILL hit you with MASSIVE penalties.