If the milk has a yellowish tint to it it’s bad. Frozen non-fat milk will last for many years. If you by chance get regular dried milk (most powdered milk is non-fat) it last much less longer, due to the fat content in it.
The Saco Foods website says their powdered non-fat milk will retain all the flavour and quality and vitamins for two years if unopened. It says vitamin quality will start to decline after three months if opened but the milk itself will be otherwise good, once opened for up to nine months.