Gravity of starter after fermentation

What is the target gravity that a starter should be after fermentation? Assuming the OG is 1.030-1.040

Sure most brewers do not measure it and it would vary according to the yeast used. Are you looking at pitching the yeast into a brew or just harvesting the yeast and storing for later use?

I am pitching the yeast into a brew. It's been 3 days on a stir plate and my gravity is 1.015, I just wanted to make sure it was completely fermented before I took it off of the stir plate.

Used light DME and Wyeast 1056

You don't need to wait for complete fermentation to pitch a starter. In fact, it's normal a good idea not to wait that long.

In a starter you want to grow healthy cells, don't worry about fermenting the wort. Most of the cells are actually generated within the first 24-48 hours. This is the end of the exponential growth phase. When the gravity is about half of the starting gravity nearly all of the cells have been produced.

For details see here:
http://woodlandbrew.blogspot.com/2012/12/yeast-cell-growth-observations.html

I made a 1.044 starter wort with 1056 Sunday and when I stepped it up tonight, the spent wort read at 1.010-12.

If your house is +65 then it's done.

An OG of 1.040 with 75% attenuation would leave you with a FG of 1.010. If your FG is 1.015 and you had 75% attenuation, your OG would have to have been 1.060. I would guess that the yeast hasn't fully attenuated after three days, but I wouldn't think that it would be very far off.