some will say it's too close.  Really, it's completely subjective.  Try it out with your current desk, and if the desk feels too small, modify your current desk or get a bigger one.  Like I said, it's all just personal preference.  I have a desk that is 38" deep, and i sometimes feel that my 22" monitor is too close.  My brother has a desk that is a lot shallower than mine (i don't know how many inches), and he has his 24" imac set up there with no problem