I am speaking from experience right now.  I have 5 desktops running in my office at work and I am not making much from them at all.  They are all good systems and they make me a few altcoins per month, but they are not worth the electric they are using.  Now if you have free electric like I do, then it is worth the time for you to do it.  If you look hard enough, you will make more money in signature campaigns than you will mining with a desktop computer.