Loose skin and stretch marks are not so much a product of the rate of weight loss, but rather the elasticity or plasticity of the skin when loss or gain occurs.
When we gain considerable size, whether through increased muscle or fat bulk, our skin is required to stretch or grow to accommodate the change. If the gain is made slowly, new skin cells will be produced. However, if the gain is made too rapidly, skin cell growth cannot keep up with the change, and our skin is consequently stretched. And if our skin is stretched past the point of its elastic limit, plastic change will be visible as stretch marks.
Similarly, when we lose considerable size, we hope that the tension of our skin will still be sufficient to hug our shape. Of course, if the change has been too great, our skin will be reduced to its resting length, and will sit loosely on our diminished frames.
Thus, the ability for our skin to accommodate us at a given size is dependent on its tension and elasticity. Younger, healthier skin is tenser and more elastic, and can therefore accommodate considerable change without evidence. However, as we age, our skin becomes stiffer, less tense, and less flexible, and we are consequently more susceptible to gaining stretch marks when we gain size.
Once the skin is stretched, there is no known natural mechanism for it to return to its former size, and surgical intervention is necessary.
So to answer your question plainly, you can lose any amount of weight within a given time-frame. If you are concerned about its being loose, then you can simply moderate fat loss with muscle gain within the bounds of your genetic potential.
I hope that helps.