I don't have any scientific studies to offer you, but my own observations are it depends on dosage and frequency. If you're repeatedly doing short, back to back type of cycles you can expect to get less results as time goes on; although you should be making progress nonetheless.
If you do a cycle, do the requesite post ancillaries, train naturally for several months, or longer, I think you'll enjoy more gains each time out. I think you're less likely to develop a resistance to most anything if you don't do it often.