The point of PWHT in alloy steel is to temper the martensite that forms in zones that have reached temperatures in which austenite forms. If this is not done, this constituent will be brittle.
Any standard will simply follow the normal post-quench tempering guidelines for the grade in question, available in reference books like the ASM Metals Handbook. Page 28.16 in the desk edition.
 A by-product of tempering is stress relief.

Thanks mcguire...Do you mean both PWHT (before in service) and post heating (immediately after welding) are necessary?

Post heating is normally carried out to drive off hydrogen after welding.  It will neither give much stress relief or temper to the material.  It can be carried out if there’s no PWHT or if PWHT is delayed, if there is a risk of hydrogen cracking, which is likely with alloy steel that easily from martensite.  15mins is not very long, 2 hours +  is usual, depending on thickness and grade.

Regards

John    www.gowelding.com

PAN
  John Dyson makes a good precautionary point about the possibility of hydrogen embrittlement. If the weld is stressed ( residual or applied )and if hydrogen has been picked up, the un-tempered martensite is quite susceptible to delayed cracking. See the same Metals handbook, desk edition page 30.28.
  However, the PWHT, which to answer your question should be right after welding, will temper the martensite. In 10 minutes at 400C the hardness of untempered martensite will drop from HRC 66 to 50. To reduce the hardness another 5 points will require another 5-10 hours.
  Stress relieving is slower. About 50% of the stresses are relieved in one hour at 400C. Going to 6 hours gives another 10%. In both processes increasing temperature is more effective. If you successfully temper and stress relieve you will also address the need for hydrogen out-gassing.