In post menopausal women without bleeding endometrial thickness >11 mm should require a biopsy to be taken to exclude endometrial cancer. 

< or =11 mm is associated with low risk for endometrial cancer.

Following article explains in more detail 

http://www-ncbi-nlm-nih-gov.ez.statsbiblioteket.dk:2048/pubmed/?term=how+thick+is+too+thick%3F+when+endometrial+thickness+prompt+biopsy

Normal thickness < 5 mm. Some even have a cut off of 4 mm for postmenopausal women assuming they are not on hormone replacement or tamoxifen. A thickness of 8 to 15 mm is acceptable for women on HRT, and a for women on tamoxifen, the ET should be < 6mm.

In post menopausal women without bleeding endometrial thickness >11 mm should require a biopsy to be taken to exclude endometrial cancer.This is a must. Those on HRT or tamoxifen may have higher cutoff.

Thanks for all of you

I have searched pubmed acording to the link of  showed by Sergey Machado-Abakumov

 endometrial+thickness+prompt+biopsy 

Here are the suggestions by Smith-Bindman R1

In a postmenopausal woman without vaginal bleeding, if the endometrium measures > 11 mm a biopsy should be considered as the risk of cancer is 6.7%, whereas if the endometrium measures < or = 11 mm a biopsy is not needed as the risk of cancer is extremely low.

In a postmenopausal woman with vaginal bleeding, the risk of cancer is approximately 7.3% if her endometrium is thick (> 5 mm) and < 0.07% if her endometrium is thin (< or = 5 mm). 

Our apporach is: with HT endometrium thick must be < 8 mm and without HT < 5 mm. Al these must be usnder investigation (hysteroscopy or endometrium byopsy) 

What about those on HRT or tamoxifen? Who knows?

our approach is, on HT Treatment acceptable endometrium Thickness is around 7mm-8mm but without HT the endometrial thickness should be of 4mm. anything above it better to go for either Hysteroscopy or Endometrium biopsy.

We have a paper published in Maturitas2013 Jun;75(2):181-90 which is available in ResearchGate.

It  appears  that  there  are  various  cut  off  values  for   normal  endometrial  thickness  in  post-menopausal  women  as  gleaned  from  the  erudite  scholars  above.

Could  race, anthropometric  variables   and  other  factors  have  a  role  to  play? It  would  be  great  to  have  a  generally-universal  cut  off  value in  which  to  initiate  further  investigation.

In a postmenopausal woman with vaginal bleeding, the risk of cancer is approximately 7.3% if her endometrium is thick (> 5 mm) and < 0.07% if her endometrium is thin (< 4 mm). 

agreed with this

In a postmenopausal woman without vaginal bleeding, if the endometrium measures > 11 mm a biopsy should be considered as the risk of cancer is 6.7%, whereas if the endometrium measures < or = 11 mm a biopsy is not needed as the risk of cancer is extremely low.

In a postmenopausal woman with vaginal bleeding, the risk of cancer is approximately 7.3% if her endometrium is thick (> 5 mm) and < 0.07% if her endometrium is thin (< or = 5 mm). 

Is there any standard of endometrial thickness under transvaginal sonography which indicates the need for further hysteroscopic diagnosis?. Available from: https://www.researchgate.net/post/Is_there_any_standard_of_endometrial_thickness_under_transvaginal_sonography_which_indicates_the_need_for_further_hysteroscopic_diagnosis [accessed Apr 19, 2017].