okay, so I'm a nerd when it comes to clinical data on this stuff... below are several links showing to what extent and how quickly HPTA suppression can occur from various androgens/SARMs. it is worth noting, that some of the dosages are far, far lower then what we see athletes use, whereas some are pretty applicable.
12 weeks of Ostarine at 3 mg/day suppressed testosterone by 38%:
http://www.ergo-log.com/enobosarmtrial.html
3 weeks of LGD 4033 at 1 mg/day suppressed testosterone by 300 ng/dl:
http://www.ergo-log.com/lgd4033.html
500 mg of testosterone reduced LH by 92% and 250 mg of testosterone reduced LH by 78%
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4266340/
2 months of Dianabol at 15 mg/day suppressed testosterone 69% and LH 50%
https://www.ncbi.nlm.nih.gov/pubmed/793272
100-200 mg/wk of nandrolone reduces LH from 5 to nearly 0 in approximately a month
http://www.ergo-log.com/decacycle.html
2 weeks of Winstrol at 10 mg/day reduced testosterone 55%:
https://www.ncbi.nlm.nih.gov/pubmed/6430603
6 months of Anavar at 2.5 mg/day cut testosterone levels in half:
https://www.ncbi.nlm.nih.gov/pubmed/8319371
Proviron:
http://www.popline.org/node/506648
12 weeks of Ostarine at 3 mg/day suppressed testosterone by 38%:
http://www.ergo-log.com/enobosarmtrial.html
3 weeks of LGD 4033 at 1 mg/day suppressed testosterone by 300 ng/dl:
http://www.ergo-log.com/lgd4033.html
500 mg of testosterone reduced LH by 92% and 250 mg of testosterone reduced LH by 78%
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4266340/
2 months of Dianabol at 15 mg/day suppressed testosterone 69% and LH 50%
https://www.ncbi.nlm.nih.gov/pubmed/793272
100-200 mg/wk of nandrolone reduces LH from 5 to nearly 0 in approximately a month
http://www.ergo-log.com/decacycle.html
2 weeks of Winstrol at 10 mg/day reduced testosterone 55%:
https://www.ncbi.nlm.nih.gov/pubmed/6430603
6 months of Anavar at 2.5 mg/day cut testosterone levels in half:
https://www.ncbi.nlm.nih.gov/pubmed/8319371
Proviron:
http://www.popline.org/node/506648